We treated 93 patients who had acute nonlymphoblastic leukemia in the first remission or chronic myelocytic leukemia in the chronic phase (median age, 30 years) with high-dose cyclophosphamide and fractionated total-body irradiation, followed by infusion of marrow from an HLA-identical sibling. To evaluate postgrafting prophylaxis for graft versus host disease, we studied these patients in a sequential, prospective, randomized trial that compared the effect of a combination of methotrexate and cyclosporine (n = 43) with that of cyclosporine alone (n = 50). All patients had evidence of sustained engraftment. A significant reduction in the cumulative incidence of grades II to IV acute graft versus host disease was observed in the patients who received both methotrexate and cyclosporine (33 percent), as compared with those who were given cyclosporine alone (54 percent) (P = 0.014). Seven patients who received cyclosporine alone acquired grade IV acute graft versus host disease, as compared with none who received both methotrexate and cyclosporine. Thirty-five of the 43 patients given both methotrexate and cyclosporine and 31 of the 50 patients given cyclosporine are alive as of this writing, at 4 months to 2 years (median, 15 months); the actuarial survival rates in the two groups at 1.5 years were 80 percent and 55 percent, respectively (P = 0.042). We conclude that the combination of methotrexate and cyclosporine is superior to cyclosporine alone in the prevention of acute graft versus host disease after marrow transplantation for leukemia, and that this therapy may have a beneficial effect on long-term survival.