Transfusion requirements of 477 patients with hematologic malignancies undergoing BMT from HLA-identical siblings were studied. The median (range) number of red cells transfused in the first, second and third months were 4 (0-32), 1 (0-39), and 0 (0-22) respectively, and the number of random donor platelet concentrates 32.5 (0-196), 0 (0-220) and 0 (0-135). On multivariate analysis, diagnosis other than acute leukemia, conditioning regimen other than cyclophosphamide-TBI, cyclosporine-methotrexate GVHD prophylaxis, and occurrence of acute GVHD increased platelet requirements significantly in the first month. Diagnosis other than acute leukemia, donor-recipient ABO incompatibility, conditioning regimen other than cyclophosphamide-TBI, and age over 18 years increased red cell requirements significantly in the first month. Platelet requirements in the second month and red cell requirements in the second and third months were increased significantly by the occurrence of acute GVHD, a diagnosis other than acute leukemia, and a conditioning regimen other than cyclophosphamide-TBI. Platelet requirements in the third month were influenced only by acute GVHD. We conclude that ABO incompatibility does not influence platelet requirements after allogeneic BMT. It affects red cell requirements in the first month along with the diagnosis, conditioning regimen, and age. However, other factors outweigh the influence of ABO-incompatibility on red cell requirements beyond the first month.