ABO incompatibility is not a barrier to allogeneic hematopoietic stem cell transplantation (HSCT). However, the impact of an ABO mismatch on the outcome of the HSCT remains controversial. We analyzed whether ABO incompatibility leads to an increased risk of early/late complications, mortality, or increased transfusion requirements. The 147 consecutive allogeneic HSCTs includes 80 ABO-identical and 25 major, 30 minor, and 12 bidirectional ABO-mismatched grafts. The four groups were balanced with respect to disease status at transplantation. Transplantation-related mortality was significantly greater (P < .01) and overall survival significantly shorter (P = 0.2) among HSCT recipients with minor ABO-mismatched grafts. The relapse rate, progression-free survival, and transfusion requirements until discharge were not different between ABO-identical and ABO-mismatched groups. Pure red cell aplasia (PRCA); (P < .0001) and delayed red blood cell (RBC) engraftment (P < .001) were more frequent in HSCT recipients with major mismatched donors. Delayed RBC engraftment was associated with posttransplantation hyperferritininemia and increased mortality risk (P = .05). The greater frequency of sinusoidal obstruction syndrome and graft-versus-host disease (GVHD) in patients with minor mismatched transplants, did not show statistical significance. In contrast severe GVHD was significantly more frequent among minor mismatched patients (P = .04). ABO-mismatched HSCT might have an unfavorable impact on transplant outcomes. Selection of ABO-compatible donors when possible, strategies to prevent and treat PRCA, modifications in transfusion practice, and effective iron chelation are among the measures that can improve transplant outcomes.