The study was undertaken to investigate the effectiveness of allogeneic bone marrow transplantation from HLA-identical siblings after preparation with busulfan and cyclophosphamide in adults with acute lymphoblastic leukemia (ALL). Thirty-nine patients aged 15 to 42 years underwent transplantation at three different centers from November 1984 through November 1990. All patients received 16 mg/kg busulfan and 120 mg/kg cyclophosphamide as preparative therapy. Cyclosporine plus methotrexate or cyclosporine plus corticosteroids with or without methotrexate were given for prevention of graft-versus-host disease (GVHD). Twelve patients died of treatment-related complications, 12 patients relapsed, and 15 patients are leukemia-free survivors. For 27 patients in group 1 (first remission, second remission, first relapse), the estimated leukemia-free survival (LFS) rate is 42.3% (95% confidence interval [CI], 22.9% to 71.7%) at 3 years. For 12 patients with more advanced disease (group 2), the 1-year LFS rate is 13.5% (95% CI, 0% to 37.1%). Chronic GVHD occurred at an estimated incidence of 63.3% and developed significantly more frequently among patients who received corticosteroids for prevention of acute GVHD. Chronic GVHD was associated with a significantly lower incidence of relapse and with improved LFS rates. LFS rate in this study is comparable to that obtained with radiation-containing regimens; however, the effectiveness of this preparative regimen in ALL requires further study.