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Background: BK polyomavirus-associated nephropathy (BKPyVAN) is an important cause of allograft dysfunction and failure in kidney transplant recipients (KTRs) and there are no proven effective treatments. Case reports and in vitro data support the potential activity of cidofovir against BK polyomavirus (BKPyV)., Methods: We report the results of a phase I/II, double-blind, placebo-controlled randomized dose-escalation trial of cidofovir in KTRs with biopsy-confirmed BKPyVAN and estimated glomerular filtration rate ≥30 mL/min. Intravenous cidofovir (0.25 mg/kg/dose or 0.5 mg/kg/dose) or placebo was administered on days 0, 7, 21, and 35, with final follow-up through day 49., Results: The trial was prematurely discontinued due to slow accrual after 22 KTRs had completed the study. Cidofovir was safe and tolerated at the doses and duration studied. The proportion of subjects with any adverse event (AE) was similar between groups (9/14 [64%] in the combined cidofovir dose groups and 6/8 [75%] in the placebo group); 84% of AEs were mild. BKPyV DNAemia reduction by day 49 was similar between groups (>1 log 10 reduction in (2/9 [22.2%] of 0.25 mg/kg group, 1/5 [20%] of 0.5 mg/kg group, and 2/8 [25%] of placebo group)., Conclusions: These preliminary results indicate that low-dose cidofovir was safe and tolerated but had no significant BKPyV-specific antiviral effect in KTRs with BKPyVAN., (© 2024 The Author(s). Transplant Infectious Disease published by Wiley Periodicals LLC.)

Background: BK polyomavirus (BKV) can cause permanent loss of allograft function due to BKV-associated nephropathy (BKVN) in kidney transplant recipients. Besides immunosuppression reduction, there are no consistently effective interventions for BKV infection. Study purpose was to define natural history of BKV infection, identify risk factors for BKV reactivation and BKVN in kidney transplant recipients, and inform the design/conduct of future clinical trials of BKV-targeted therapeutics., Methods: We conducted a multicenter prospective observational study of incident kidney transplant recipients at six U.S. transplant centers. Participants were monitored every 4 weeks for BKV reactivation and followed for up to 24 months post-transplant. We used regression models (logistic, survival, mixed models) to study relationships between BK viremia/BKVN, clinical characteristics, and allograft function., Results: We enrolled 335 participants. Fifty-eight (17%) developed BK viremia, 6 (2%) developed biopsy-proven BKVN, and 29 (9%) developed suspected/presumed BKVN (defined as BKV viral load > 10,000 copies/mL without biopsy). Male donor sex was associated with lower odds for BK viremia, whereas recipient Black race was associated with two-fold increased odds for BK viremia. Recipient female sex was associated with more rapid clearance of BK viremia. Persistent BK viremia/BKVN was associated with poorer allograft function by 24 months post-transplant., Conclusions: We identified multiple donor and recipient demographic factors associated with risk for BKV infection and poorer allograft function by 24 months post-transplant. This may help design future clinical trials of therapies to prevent or mitigate the deleterious impact of BKV reactivation on kidney transplant outcomes., (© 2024 Wiley Periodicals LLC.)

BK viremia is endemic among kidney transplant recipients (KTRs). Incidence, risk factors, outcomes, and clinical management of detectable versus high BK viremia have not been considered previously in KTR in the modern era. This observational study examined KTR transplanted between January 1, 2009 and December 31, 2016. Any BK viral load in the serum constituted detectable BK viremia and ≥10 3 copies/ml constituted high viremia. Among 1193 KTRs, the cumulative probability of developing detectable and high BK viremia within 2 years posttransplant were 27.8% and 19.6%, respectively. Significant risk factors for detectable BK viremia included recipient age (HR 1.02 [95% CI: 1.01, 1.03]) and donor age (HR 1.01 [95% CI: 1.00, 1.02]). Recipient age also predicted high BK viremia (HR 1.02 [95% CI: 1.01, 1.03]), whereas White race (HR 0.70 [95% CI: 0.52, 0.95]), nondepleting induction therapy (HR 0.61 [95% CI: 0.42, 0.89]), and delayed graft function (HR 0.61 [95% CI: 0.42, 0.88]) were protective. Mean estimated glomerular filtration rates were 4.28 ml/min/1.72 m 2 (95% CI: 2.71, 5.84) lower with detectable BK viremia. Although low viral load was usually not acted upon at first presentation, antiproliferative dose reductions were the most common initial management. BK viremia remains a common early complication in a modern cohort of KTRs. These findings highlight the benefit of early BKV monitoring in addition to intensive clinical management. Clinical responses beyond first positive BK viremia tests, and their implications for graft outcomes, merit further investigation., (© 2022 Wiley Periodicals LLC.)

Objectives: BK polyomavirus (BKV) causes two distinct complications after transplantation, hemorrhagic cystitis (BKV-HC) after hematopoietic stem cell transplantation (HSCT), and BKV-associated nephropathy (BKVAN) after kidney transplantation (KT). Although fluoroquinolones show efficacy against BKV proliferation in vitro, the clinical effect remains uncertain; thus, we performed meta-analysis to assess its efficacy in the prophylaxis., Methods: Articles published before March 2020 were searched from PubMed, the Cochrane Library, ISRCTN registry, and ClinicalTrials.gov. Primary outcomes were BKV-HC after HSCT and BKVAN after KT. Secondary outcomes were BK viremia, viruria after KT, and fluoroquinolone-related adverse events., Results: Three trials with 281 patients post-HSCT and 11 trials with 1882 patients post KT were included as for prophylaxis. Fluoroquinolone prophylaxis did not show effects on BKV-HC (OR 0.54, 95% CI 0.13-2.25), BKVAN (OR 0.74, 95% CI 0.35-1.55), and BK viremia (OR 0.79, 95% CI 0.49-1.28), but significantly decreased BK viruria (OR 0.64, 95% CI 0.45-0.91). Fluoroquinolone prophylaxis was associated with the higher percentage of fluoroquinolone-resistant infection among identified bacteria (OR 2.38, 95% CI 1.16-4.88), but the incidence of fluoroquinolone-resistant infection was similar (OR 1.15, 95% CI 0.71-1.86), due to the decrease of infection itself (OR 0.52, 95% CI 0.34-0.81)., Conclusions: This meta-analysis showed that fluoroquinolones did not prevent BKV-HC after HSCT or BKVAN after KT, although the effect against BKV-HC should be further investigated by randomized controlled trials. Fluoroquinolones could reduce the rate of BK viruria to some extent but may not have clinically sufficient effects., (© 2020 Wiley Periodicals LLC.)

Background: Polyomavirus-associated nephropathy (PVAN) remains a relevant complication following kidney transplantation with allograft loss rates of up to 50%. Reduction in overall immunosuppression is a cornerstone of therapy, whereas no specific antiviral regimen has shown conclusive benefit to date. The present case series demonstrates the efficacy of a dual therapeutic approach with low-dose cidofovir and conversion to mTOR-based immunosuppression in PVAN., Methods: Patients with biopsy-proven PVAN having received low-dose cidofovir (0.25 mg/kg) according to the Tübingen Cidofovir Protocol and been converted to mTOR-based immunosuppression were analyzed retrospectively., Results: Twenty-three patients with a median follow-up of 2.24 [IQR 1.55-5.01] years were included in the analysis. Median time to PVAN diagnosis was 268 [IQR 153-869] days after transplantation. Polyomavirus clearance from plasma was achieved in 78% of patients after a median of 118 [IQR 76-293] days. Of the 23 patients, nine patients (39%) lost their allograft function during follow-up, but only three of these (13%) due to PVAN. Fourteen patients (61%) stabilized or improved allograft function. The cidofovir protocol allowed for specific antiviral therapy without adverse nephrotoxicity, even in patients with low allograft function., Conclusions: Low-dose cidofovir and conversion to mTOR-based immunosuppression allow for effective virus clearance and preservation of allograft function in a high proportion of patients with PVAN and progressive allograft dysfunction and may prolong allograft survival in these patients., (© 2019 Wiley Periodicals, Inc.)

Background: BK polyomavirus associated nephropathy (BKPyVAN) is a significant clinical issue in kidney transplant (KT) recipients. No specific therapy is currently available, although treatment with leflunomide may be part of the therapeutic strategy. Here, we sought to examine the impact of leflunomide on the evolution of BKPyVAN., Methods: This was an observational retrospective study conducted in 3 French transplant centers. KT recipients who developed BKPyVAN and received leflunomide after failure of other treatment approaches were deemed eligible. Graft function, viral clearance, patient survival, rejection rates, treatment tolerability, and immunosuppression levels served as the main outcome measures., Results: A total of 55 patients were included. Treatment with leflunomide was started after a mean of 1.4 ± 4.1months  after BKPyVAN diagnosis. Between the introduction of leflunomide and the end of follow-up, creatinine levels increased by 31 ± 118% (P = 0.04), whereas viremia decreased by 79 ± 37% (P < 0.001). Blood viral clearance was observed in 76% of the study patients. Rejection episodes occurred in 33% of the participants. Eleven patients lost their graft (9 of which because of BKPyVAN). Ten patients developed adverse effects and 3 discontinued leflunomide., Conclusion: We cannot conclude about the exact place of leflunomide in the therapeutic strategy of BKPyVAN. It may be a part of the therapy to promote BK polyomavirus clearance in cases of BKPyVAN who fail to improve after immunosuppression lowering alone. Unfortunately, a significant decline in renal function and high rejection rates remain major clinical challenges., (© 2019 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Introduction: BK polyomavirus (BKPyV) continues to impact renal transplant recipients (RTR). The New England BK Consortium aims to jointly optimize screening and management of BKPyV., Methods: Our first project was to survey centers' BKPyV screening protocols and compare them to consensus guidelines., Results: Thirteen of 15 centers (86.7%) returned the survey. Only two center reported using monitoring parameters that were in line with consensus guidelines for BKPyV screening, while the majority of centers reported less intensive methods and shorter duration. One center reported performing renal biopsies in all patients with plasma viral loads >10 000 copies/mL, while all other centers only perform for-cause biopsies. For presumptive nephropathy, 11 centers recommend a biopsy for confirmation. For management of documented BKPyV-associated nephropathy, 12 centers propose further immunosuppression reduction. Nine centers report CNI dose reduction as their primary treatment. More than half of centers surveyed reported use of leflunomide, cidofovir or intravenous immunoglobulin., Conclusions: There was a large variance in BKPyV screening and management strategies among centers. Due to these results, all participating centers agreed to implement uniform screening and aim to optimize management protocols., (© 2018 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Introduction: BK polyomavirus can lead to hemorrhagic cystitis (BKPyV-HC) in allogeneic stem cell transplantation and therefore to increased morbidity. No causal therapy has been established yet. Cidofovir (CDV) is a nucleotide analog of cytosine that is active against various DNA viruses and it has been described for therapy of BKPyV-HC using 2 admission routes: intravenous and intravesical., Methods: We performed a systematic review regarding the comparison of intravenous or intravesical cidofovir in the treatment of BKPyV-HC following adult allogeneic stem cell transplantation. Since there is a lack of randomized controlled trials, we considered all kinds of studies for this review. Due to heterogeneity of the data, we were not able to perform a meta-analysis, so the results are shown descriptively., Results: The literature search for primary studies yielded 232 results. Finally, 9 studies where considered which included a total of 189 adult patients with BKPyV-HC after allogeneic stem cell transplantation. We could only identify retrospective studies for this review. A total of 172 patients received intravenous CDV, 17 patients received intravesical CDV, and 2 patients received CDV in both admission routes. In 68.0% of the cases, a complete response for intravenous CDV was documented and in 88.2% for intravesical CDV. Interestingly, no kidney toxicity was mentioned in intravesical CDV. 9.3% of the intravenously treated patients had renal failure., Conclusion: There is only weak evidence for the use of CDV. The intravesical admission route should be further investigated because of a good toxicity profile., (© 2018 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

BK polyomavirus mostly manifests as polyomavirus-associated nephropathy (PyVAN) in kidney transplant patients and polyoma virus-associated hemorrhagic cystitis (PyVHC) in bone marrow transplant patients. PyVHC in kidney transplant patients is only reported in four cases in the literature. Our patient had severe hemorrhagic cystitis without renal involvement. We postulate that our patient's exposure to ifosfamide and radiation 8 years prior transplantation might predispose him to this disease., (© 2017 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Human polyomavirus-7-associated rash and pruritus (PVARP) is a chronic superficial viral skin infection, which primarily impacts immunocompromised individuals. We report on a case of PVARP in a lung transplant recipient. Our patient developed symptoms 13 years after being on his immunosuppressive regimen, with an insidious course of progressive gray lichenification with marked islands of sparing and quality of life-altering pruritus. Treatment for PVARP is not established; however, topical cidofovir combined with immunomodulation may offer sustained therapeutic benefit., (© 2017 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

JC polyomavirus (JCPyV)-associated nephropathy (JCPyVAN) occurs in <3% of PVAN cases after renal transplantation. We report the first confirmed case to our knowledge of JCPyVAN diagnosed by kidney biopsy in the early 6 months post transplant in Thailand. In this case report, recovery of renal allograft function was not observed after reduction of immunosuppressive agents and administration of intravenous immunoglobulin and cidofovir. Despite persistent JCPyV viruria, no significant further decline in allograft function was documented at 15 months post transplant., (© 2017 The Authors. Transplant Infectious Disease Published by John Wiley & Sons Ltd.)

Background: BK polyomavirus virus (BKPyV) screening and immunosuppression reduction effectively prevent graft loss due to BKPyV-associated nephropathy (BKPVAN) during the first year after transplantation. The aim of our study was to evaluate the impact of this infection during longer follow-up periods., Methods: We reviewed the outcome of our screening and immunosuppression reduction protocol in 305 patients who received a kidney transplant between March 2008 and January 2013. Quantitative BKPyV DNA surveillance in plasma was performed at 1, 2, 3, 6, 9, and 12 months after transplantation. Patients with significant viremia and/or biopsy-proven BKPVAN were treated with immunosuppression reduction and leflunomide., Results: During the first post-transplant year, 24 patients (7.9%) developed significant viremia at a median time of 95 days, and 18 patients had BKPVAN; 23 of the 24 (7.5%) were treated according to our protocol (group BKV+); 225 patients (73.8%) did not develop any BK viremia (group BKV-). Allograft function was similar in both groups at 1 month post transplantation (P=.87), but significantly worse at 1 year in the BKV+ group (P=.002). Thereafter, kidney function stabilized in the BKV+ group and no differences in patient and graft survival were seen between the groups after a median follow-up of 4 years., Conclusions: We confirm the early occurrence of BKPyV replication after transplantation and the short-term decline in renal function. However, early detection of BKPyV replication, prompt diagnosis, and reduction in immunosuppression may offer long-term benefits for graft function., (© 2017 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

To date, only one case of BK polyomavirus (BKPyV) encephalitis combined with transplant-associated thrombotic microangiopathy has been reported in an hematopoietic stem cell transplantation (HCT) recipient. We report the case of an HCT recipient who developed thrombotic microangiopathy and subsequent BKPyV encephalitis. She died despite treatment with cidofovir, ciprofloxacin, and intravenous immunoglobulin without improvement in mental status. Early suspicion of BKPyV encephalitis in an HCT recipient presenting with altered mental status and hemorrhagic cystitis is important., (© 2016 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Background: Although high-dose steroid therapy has been attempted for the management of clinically suspected allograft rejection, before testing for BK viral activity or acute cellular rejection accompanied by BK polyomavirus nephropathy, its long-term outcome remains unknown. We investigated the impact of high-dose steroids on BK viral activity and long-term graft outcomes in patients with BK viremia., Methods: The study population comprised 144 kidney transplant recipients with BK viremia. They were divided into 2 groups based on the amount of steroids administered: low-dose group (<2 g, n=123) or high-dose group (≥2 g, n=21)., Results: The baseline serum BK viral loads were 5.4±1.1 log cp/mL in the low-dose group and 6.0±1.3 in the high-dose group (P=.054). These changed to 5.2±1.3 and 6.1±1.4, 1 month after steroid treatment (P=.03) and 4.9±1.3 and 5.9±1.4 at 2 months (P=.033), respectively. From 3 months to 1 year, the serum BK viral titers were not different between groups. Kaplan-Meier analyses demonstrated that the rates of the decline of graft function and graft failure were higher in the high-dose group (P=.02 and P=.04, respectively). High-dose steroids (P=.012, hazard ratio [HR] 5.04, 95% confidence interval [CI] 1.42-17.85) and log serum BK viral load at 2 months after steroid treatment (P=.042, HR 1.52, 95% CI 1.02-2.28) were independent risk factors for the decline of graft function., Conclusion: High-dose steroids induced BK viral activation and subsequently resulted in poor long-term graft function and early graft failure in patients with BK viremia., (© 2016 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Background: BK viral nephropathy is an increasingly recognized cause of early allograft loss in kidney transplantation. This study aimed to determine whether a sirolimus (Sir)-based calcineurin inhibitor-sparing regimen is associated with a lower incidence of BK viremia., Methods: This was a single-center retrospective study. Patients were either on tacrolimus (Tac)-based or on Sir-based immunosuppression. Conversion from Tac to Sir occurred at or after 3 months if patients were <62 years of age, had calculated panel reactive antibodies of <20%, and did not have acute early rejection., Results: Incidence of clinically significant BK viremia was 17.9% in the Tac group and 4.3% in the Sir group. Cox regression multivariate analysis showed that male gender (hazard ratio [HR] = 2.87) and switch to Sir (HR = 0.333) impacted the incidence of BK viremia. Kaplan-Meier analysis showed a higher BK-free survival in the Sir group. A trend was seen toward shorter time to resolution of BK viremia and lower peak viremia in the Sir group. Patients on Sir had a higher estimated glomerular filtration rate at each time point; 34% of patients discontinued Sir because of side effects., Conclusion: Conversion to Sir-based maintenance immunosuppression at or about 3 months after kidney transplantation correlates with a lower incidence of BK viremia., (© 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Background: In renal transplantation, BK virus infection can result in significant graft nephropathy and loss. While reduction in immunosuppression (IS) is considered standard therapy, adjunct agents may be warranted. Data are suggestive of a possible role of cidofovir for the management of BK. This study aims to describe the course of BK viremia (BKV) in a large cohort of renal transplant patients receiving adjunct cidofovir., Methods: We evaluated kidney and kidney-pancreas recipients who received cidofovir combined with reduced IS for management of high-level BKV or BK virus nephropathy (BKVN). We examined the rate and timing of BKV clearance, and performed a multivariate analysis to identify risk factors associated with long-term (>6 months) viremia., Results: In total, 75 patients received a median of 13 doses of cidofovir in conjunction with reduced IS; 32 patients (43%) had short-term BKV (≤6 months), and 43 (57%) had long-term BKV. Overall, 53 of 75 patients (71%) eventually cleared BKV at a median of 4.2 months (interquartile range 2.1-9.3 months). Independent factors associated with long-term BKV included older age (odds ratio [OR] 1.1, P = 0.02), delayed graft function (OR 31.4, P = 0.01), and higher peak BKV (OR 12.8, P = 0.02), while BKV reduction by at least 1 log(10) copies/mL at 1 month of treatment was associated with clearance within 6 months (OR 49.3, P < 0.01). Patients with earlier clearance maintained stable graft function and no graft losses, while long-term BKV was associated with a 15% decline in estimated glomerular filtration rate., Conclusions: Adjunct cidofovir resulted in preservation of renal function when viral clearance occurred within 6 months of initiation. This retrospective review defines factors predicting response to cidofovir in conjunction with reduced IS for BKVN or high-level BKV. Still, considering cost, frequency of administration, and treatment duration, a randomized trial is necessary to define the exact utility of cidofovir in the setting of BK virus infection., (© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

We report the case of a 39-year-old male patient who died of severe BK virus (BKV) pneumonia 168 days after hematopoietic stem cell transplantation (HSCT) for acute lymphoblastic leukemia. After suffering from BKV-associated late-onset hemorrhagic cystitis (HC) with long-term sustained BKV viremia, he died of rapidly progressive pneumonia. On autopsy, numerous viral intranuclear inclusions were seen in his lungs and bladder. An immunohistochemical examination of his lungs was positive for simian virus 40. Based on these pathological results and the high sustained BKV viral load in his blood, we reached a diagnosis of BKV pneumonia. Viral infection can occasionally become life threatening among HSCT recipients. It is widely known that BKV can cause late-onset HC, but BKV-associated pneumonia is rare. Because of its rapid progression and poor prognosis, it is difficult to make an antemortem diagnosis of BKV pneumonia. A treatment strategy for BKV pneumonia also needs to be formulated. Similar to other viral pathogens, BKV can cause pneumonia and the clinician should therefore be aware of it in immunocompromised patients., (© 2012 John Wiley & Sons A/S.)

Unlabelled: BK virus (BKV) nephropathy is a common viral infection in renal transplant patients, with a prevalence of 1-9% at approximately 12 months after surgery. While it is widely agreed that reduction of immunosuppression should be the first intervention after diagnosis of BKV infection, there is no consensus on whether calcineurin inhibitors or antiproliferative drugs should be reduced first. Furthermore, target levels of immunosuppressive drugs are poorly defined, as are criteria for replacing one immunosuppressive agent with another., Results: We report our series of 15 renal transplant patients who underwent surgery between September 2004 and March 2010 and who developed BKV infection. The first 8 patients were treated with reduction of immunosuppression; 7 of these patients received cidofovir and 6 received intravenous immunoglobulin. The remaining 7 renal transplant recipients received mammalian target of rapamycin inhibitors (imTOR). In this group, we observed faster and more efficacious BKV clearance in plasma and urine and a steady improvement in allograft function, with no episodes of acute allograft rejection during follow-up. The polymerase chain reaction assay for BKV in urine became positive in 2 patients in whom imTOR were stopped due to severe side effects., Conclusions: The use of imTOR should be considered a first step in the treatment of renal transplant recipients with BKV infection. In our experience, this change in treatment was safe and resulted in viral clearance., (© 2011 John Wiley & Sons A/S.)

BK virus (BKV) reactivation occurs in 50% of allogeneic hematopoietic stem cell transplantation (allo-HSCT) recipients. Standardized antiviral management of BKV infection has not been established. In order to develop a uniform guideline, a treatment algorithm for the management of symptomatic BKV replication was implemented for our allo-HSCT population. This is a retrospective analysis of patients treated according to the protocol between January 2008 and January 2009. Eighteen patients developed symptomatic BKV replication a median of 43 days after allo-HSCT. All patients had BK viruria and 12 patients had BK viremia in addition to viruria. Patients with isolated viruria were treated with intravenous (IV) low-dose cidofovir (0.5-1mg/kg IV weekly) until symptom resolution. In patients with BK viremia, therapy was continued until virological clearance was achieved in the blood. Four patients also received intravesical instillation of cidofovir per physician discretion. Thirteen of 18 (72%) patients with viruria and 8 of 12 (75%) patients with viremia responded to treatment. Three patients developed transient renal dysfunction. Low-dose cidofovir is safe and effective in allo-HSCT recipients. In absence of randomized prospective data, an institutional algorithmic protocol removes variance in practice pattern and derives data that may be used for research and improved patient care., (© 2010 John Wiley & Sons A/S.)

A hematopoietic stem cell transplant recipient developed a mucosal herpes simplex virus-1 (HSV-1) infection while under acyclovir (ACV) treatment (HSV was later shown to be resistant to ACV). Concomitantly, the patient presented a hemorrhagic cystitis (HC) due to polyomavirus BK, for which intravenous cidofovir (CDV) was prescribed. The patient benefited from the broad-spectrum anti-DNA virus activity of CDV, and not only the HC resolved without signs of nephrotoxicity but also the HSV-1 lesions disappeared. This is the first report describing the effect of CDV on 2 simultaneous and unrelated DNA viral infections in an immunosuppressed transplant recipient. In addition, we describe here that this HSV-1 isolate possesses a unique phenotype and genotype.

We report the case of an 18-year-old patient who received an allogeneic bone marrow transplant from an HLA-identical unrelated donor for a Ph+ acute lymphoblastic leukemia, in his third complete remission. Cyclophosphamide and busulfan were used as conditioning treatment. Acute graft-versus-host disease developed on day +9, and the response to adequate treatment (steroids) was favourable. On day +45 the patient developed an acute severe haemorhragic cystitis, and BK polyomavirus was demonstrated in urine samples using electron microscopy and polymerase chain reaction. Urinary symptoms did not improve in spite of palliative treatment, but a response was evident after 2 weeks of cidofovir treatment.