Seventeen patients with aplastic anemia or acute leukemia received transplants from donors who had major ABO incompatibilities. Antibody titers were decreased by plasma and whole blood exchanges prior to marrow infusion. All 17 patients were successfully engrafted, and there was one possible rejection in the patient with the highest pretransplant anti-A IgG titer. Nine of 17 patients are currently alive. A review was carried out of transplants performed in Seattle between HLA-matched siblings with aplastic anemia and leukemia. Two hundred forty-six evaluable patients with ABO-compatible donors were compared with 46 with minor ABO-incompatible donors. There was no effect of minor ABO incompatibility on graft rejection, incidence and severity of graft-versus-host disease, or survival.