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Background: Post‐transplant cyclophosphamide (PTCY) alone as graft‐versus‐host disease (GVHD) prophylaxis may avoid/reduce short‐ and mid‐term toxicities of drugs commonly used for GVHD prophylaxis, accelerate immune reconstitution after the graft to decrease infections and facilitate the early integration of adjunct maintenance therapies to prevent relapse. Objective: A prospective phase 2 study was designed in order to assess the feasibility and safety of PTCY as a sole GVHD prophylaxis in adult patients receiving a Baltimore‐based reduced‐intensity conditioning (RIC) peripheral blood (PB) allogeneic hematopoietic stem cell transplantation (Allo‐HSCT) with a matched donor. Study design: Patients were planned to be included stepwise up to 59 evaluable PTCY recipients, in order to be able to stop the protocol in case of excessive corticosteroid resistant grade 3–4 severe acute GVHD (aGVHD). Because a high incidence of grade 2–4 aGVHD was observed after analysis of the first 27 patients, the protocol was amended to test the addition of 1 day of anti‐thymoglobulin to PTCY. In spite of this, the trial had to be stopped after 38 treated patients, because of an unacceptable rate of grade 3–4 aGVHD. Donors were matched related to 12 patients and unrelated to 26. Results: With a median follow‐up of 29.6 months, 2‐year overall, disease‐free and GVHD‐free relapse‐free (GRFS) survivals were respectively 65.4%, 62.1% and 46.9%. Cumulative incidences of grade 2–4 and 3–4 aGVHD at day 100 were 52.6% and 21.1%, respectively, while that of moderate/severe chronic(c) GVHD was 15.7% at 2 years. Addition of ATG to PTCY did influence neither aGVHD, cGVHD nor GRFS. Conclusion: Despite paradoxically good survivals, especially GRFS, this study failed to demonstrate that PTCY (± ATG) alone can be used for Baltimore‐based RIC PB Allo‐HSCT with matched donors. Other combinations should be tested to try and avoid long‐term use of immunosuppressive drugs following Allo‐HSCT in this setting. [ABSTRACT FROM AUTHOR]