Fifty-one patients with acute nonlymphocytic leukemia (16 with end-stage disease, 17 in second or third remission or in early relapse, and 18 in first remission) were given infusions of HLA-identical sibling marrow after cytoreduction with high doses of busulfan and cyclophosphamide. Actuarial two-year survival rates were 0 per cent, 29 per cent, and 44 per cent, respectively. Twelve patients are still alive and in remission after 327 to 1488 days, with 10 surviving beyond two years. Acute graft-versus-host disease and viral pneumonia were the major causes of death. Leukemic cells failed to clear in one patient with end-stage disease, and a relapse with meningeal leukemia occurred in another. Only one other relapse was seen--in a patient given a transplant during a third remission. Survival was favorably affected by younger age and transplantation during first remission. We conclude that high-dose chemotherapy with busulfan and cyclophosphamide, followed by allogeneic-marrow transplantation, can produce long-term remission of acute leukemia. Chemotherapy with high-dose busulfan and cyclophosphamide before transplantation provides an effective alternative to cyclophosphamide and total-body irradiation before transplantation for the treatment of acute nonlymphocytic leukemia.