Forty-seven patients with high risk acute lymphoblastic leukemia (ALL) received an allogeneic (allo) or autologous (auto) bone marrow transplant (BMT). Patients in both groups were comparable in terms of age, initial presentation of ALL and induction chemotherapy. Allo patients were transplanted earlier (median 3 months after CR) than auto patients (median 6.5 months after CR). Auto patients received more consolidation chemotherapy before BMT. All patients received total body irradiation 2.2 Gy/day x 5 days after cyclophosphamide 60 mg/kg x 2 (18 allo and five auto) or melphalan 140 mg/m2 (seven allo and 17 auto). Prevention of graft-versus-host disease (GVHD) was by conventional immunosuppression in 17 patients and T cell depletion in eight. Seven patients (28%) developed moderate to severe acute GVHD. Auto marrow was treated in vitro in each case. Seven patients died in CR from BMT complications (five allo and two auto). The probability of relapse was 9% for patients receiving allo BMT and 52% for patients receiving auto BMT (p less than 0.01). The disease-free survival was 71% for allo BMT and 40% for auto BMT (p = NS). Early BMT is an effective form of consolidation for high risk patients with ALL in first CR. An allogeneic anti-leukemia effect was demonstrated in this study.