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Background. Parvovirus B19 infection causes severe cytopenia and can mimic a leukemic relapse or therapy-induced cytopenia in patients with hematologic malignancies. We evaluated the complications of parvovirus B19 infection, including delays in the scheduled course of chemotherapy, in children with acute lymphoblastic leukemia (ALL). Methods. Consecutive bone marrow samples were collected from 117 children with ALL and were analyzed for parvovirus B19 DNA by polymerase chain reaction. Clinical and laboratory data were collected from the Nordic Childhood Leukemia Registry and from medical records. Results. Among the 117 children with ALL, 18 (15%) were found to be parvovirus B19 DNA positive. The infection was suspected on clinical grounds in only 1 of these 18 patients. Patients with viremia at diagnosis or during therapy for infection had lower viral loads (median viral load, 7 × 104 copies/mL) than did those who became viremic during maintenance therapy (median viral load, 2 × 108 copies/mL). The former group also had fewer clinical complications. Indeed, when parvovirus B19 DNA was present during the maintenance treatment, the number of complications (including cytopenia) increased, causing significantly longer periods without chemotherapy (median duration without chemotherapy, 59 days vs. 30 days; P ⩽ .05) and a higher number of blood transfusions (P = .018) in parvovirus B19 DNA-positive patients than in parvovirus B19 DNA-negative patients. Conclusions. Children with ALL who were infected with parvovirus B19 became cytopenic, leading to reduced treatment intensity and to complications during treatment. Screening for parvovirus B19 DNA by quantitative polymerase chain reaction in pediatric patients with ALL and unexplained cytopenia is suggested. [ABSTRACT FROM AUTHOR]