This study reviews results of a radiation-free preparative regimen consisting of busulfan and cyclophosphamide in 65 unrelated allogeneic bone marrow transplant recipients. Thirty-eight patients had chronic myelogenous leukemia (17 patients chronic phase, 13 patients accelerated phase, eight patients blast phase), 19 patients had acute leukemia (second complete remission or relapse) and eight patients had myelodysplasia. The patients were transplanted at four different medical centers from July 1988 to November 1992. Ages ranged 4-48 years (median 32). Fifty-seven patients received busulfan 16 mg/kg and cyclophosphamide 120 mg/kg, and eight received busulfan at doses between 15 and 17 mg/kg and cyclophosphamide at doses 100-200 mg/kg as preparative regimens. All patients received cyclosporine for graft-versus-host disease prophylaxis; in addition 46 patients received corticosteroid, 38 methotrexate, six anti-CD5 ricin A-immunotoxin, and four T cell-depleted bone marrow. Median follow-up of survivors was 53 months (range 15-68 months). Four year actuarial survival was 24 +/- 12%. Four-year survival based on disease was 29 +/- 27% for chronic myelogenous leukemia (CML) in chronic phase, 20 +/- 9% for chronic myelogenous leukemia in accelerated phase, 0% for chronic myelogenous leukemia in blast phase, 32 +/- 40% for acute leukemia, and 38 +/- 34% for myelodysplasia. Actuarial survival was 66 +/- 40% in patients age < 20 years, vs 23 +/- 13% for patients ages 20 to 40, and 10 +/- 14% for patients age > 40 years. Fifty patients (88%) engrafted. Graft failure occurred in eight patients. Acute graft-versus-host disease grade II-IV occurred in 36 (72%). Two patients relapsed after engraftment with the donor cells and died of leukemia within a month of relapse. The most common causes of death were graft-versus-host disease (37%), and transplant-related toxicity (59%); relapse (4%) was a rare cause of death. Busulfan/cyclophosphamide is an effective preparative regimen in unrelated bone marrow transplantation permitting adequate engraftment and a low relapse rate. Best results are observed in patients less than 20 years old.