We retrospectively analyzed transfusion requirements within the first 100 d among allogeneic haematopoietic cell transplantation (HCT) recipients with haematological malignancies given either myeloablative (n = 1353) or nonmyeloablative conditioning (n = 503). We confirmed that myeloablative recipients required more platelet and red blood cell (RBC) transfusions than nonmyeloablative recipients (P < 0·0001 for both). Myeloablative patients given peripheral blood stem cells required less platelet transfusions (P < 0·0001) than those given marrow while RBC transfusion requirements did not differ significantly. Subsequent analyses were restricted to nonmyeloablative recipients. Platelet and RBC transfusions were less frequent among related compared to unrelated recipients (P < 0·0001 for both), with comparable median numbers of transfused units. Major/bidirectionally ABO-mismatched recipients required more RBC transfusions than ABO-matched recipients (P = 0·006). Rates of graft rejection/failure, grades II–IV acute and chronic graft-versus-host-disease (GVHD), 2-year relapse, 3-year survivals and non-relapse mortality were comparable among ABO-matched, minor-mismatched, and major/bidirectionally mismatched recipients (P = 0·93, 0·72, 0·57, 0·36, 0·17 and 0·79, respectively). Times to disappearance of anti-donor IgG and IgM isohemagglutinins among major/bidirectionally ABO-mismatched recipients were affected by magnitude of pre-HCT titres (P < 0·001 for both) but not GVHD (P = 0·71 and 0·78, respectively). In conclusion, nonmyeloablative recipients required fewer platelet and RBC transfusions and among them, both unrelated and major/bidirectionally ABO-mismatched recipients required more RBC transfusions. ABO incompatibility did not affect nonmyeloablative HCT outcomes.