In 25 patients receiving allogeneic bone marrow transplants methotrexate was used to prevent acute graft-versus-host disease (GVHD). Acute GVHD, grades 2 to 4, developed in only 5 (20%) of the patients. The incidence of acute GVHD in other series of recipients of bone marrow transplants has ranged from 5% to 76%. A review of the literature suggests that this variation cannot be completely accounted for by age, type of disease treated by transplantation or type of GVHD prophylaxis. However, transfusion of allogeneic lymphocytes that have not been completely inactivated by irradiation (e.g., in platelet and granulocyte preparations) and inadequate isolation-decontamination procedures may increase the probability of GVHD following bone marrow transplantation.