Patients with acute nonlymphoblastic leukemia (ANL) in first remission (n = 38) or chronic myelocytic leukemia (CML) (n = 55) were given cyclophosphamide and total body irradiation, followed by marrow infusion from HLA-identical siblings. To evaluate postgrafting prophylaxis for acute graft-versus-host disease (GVHD), the patients were randomized to receive either methotrexate and cyclosporine (n = 43) or cyclosporine alone (n = 50). Methotrexate/cyclosporine significantly reduced the incidence and severity of acute GVHD, and improved early survival. This report updates the results with a 3.0 to 4.5 year follow-up. Methotrexate/cyclosporine did not interfere with sustained hematopoietic engraftment, although granulocyte recovery to 1,000/microL was delayed by five days on the average. The incidence of chronic GVHD was identical in the two groups (26% v 24%). Disease-free 3-year survival was slightly better in the methotrexate/cyclosporine group (65% v 54%), but this benefit was restricted to patients with CML (73% v 54%), while no improvement was seen in patients with ANL (41% v 41%). In contrast to patients with CML (relapse rates 8% v 9%), the early survival benefit among patients with ANL given methotrexate/cyclosporine was offset by an increase in leukemic relapses (29% v 16%).