Simple SummaryGraft-versus-host disease (GVHD) remains a main cause of morbidity and mortality in patients receiving allogeneic hematopoietic stem cell transplantation. Among GVHD prophylaxis regimens, antilymphocyte serum (ATG/ATLG) has been widely used in both unrelated donor transplant and with HLA–identical sibling donation from peripheral blood stem cells, at the cost of increased opportunistic infections, as well as relapses for delayed immune reconstitution. Moreover, there are differences in dosage and formulation with ATG which make difficult to compare results. We performed a retrospective single-center analysis on a cohort of 226 patients receiving ATG at a fixed dose of 5 mg/kg in unrelated donor transplants. GVHD’s cumulative incidence was 29.9% and 29.8%, for acute and chronic GVHD, respectively. We recorded relapse incidence and infection rates in line with other GVHD prophylaxis regimens. Thus, we suggest that this low dose of ATG could be used as an effective GVHD prophylaxis without a significant worsening of other transplant outcomes.AbstractGraft-versus-host disease (GVHD) is one of the most important complications of allogeneic hematopoietic stem cell transplantation. Rabbit antilymphocyte serum (ATG/ATLG) is recommended for GVHD prophylaxis, while its appropriate dosing is debated. We performed a retrospective single-center study to examine the outcome of patients receiving ATG at the dose of 5 mg/kg as GVHD prophylaxis for unrelated donor (URD) HSCT. We collected data from all consecutive adult patients with hematological malignancies who had undergone allogeneic HSCT from URDs at the Stem Cell Transplant Center of the Città della Salute e della Scienza Hospital of Torino between July 2008 and July 2021. The primary aim was to ascertain the cumulative incidence (CI) for acute GVHD (aGVHD) and chronic GVHD (cGVHD); the secondary aim was to ascertain the CI for NRM (Non-Relapse Mortality) and RI (Relapse Incidence), as well the overall survival (OS) and infection incidence within 30 days of transplantation. We included in the analysis 226 patients who collectively underwent 231 HSCTs. The CI of grade II–IV aGVHD was found to be 29.9%, while that of moderate to severe cGVHD was 29.8%. The CI of NRM recorded at 1, 2, and 3 years after transplant was 18.2%, 19.6%, and 20.2%, respectively. The CI of RI at 1, 2, and 3 years from transplant was recorded to be 17.8%, 21.0%, and 21.6%, respectively. The median follow-up was 56 months, while the median OS for the whole cohort was not established; the OS at 1, 3, and 5 years from transplant was 69.6%, 59.3%, and 57.2%, respectively. We registered 88 bacteremias in 82/231 patients (35.5%), while invasive fungal infections occurred in 12/231 patients (5.2%). Our study suggests that the use of ATG at 5 mg/kg is highly effective in limiting the occurrence of both aGVHD and cGVHD, ensuring a low NRM, RI, and infection incidence.