Bone marrow transplantation (BMT) can cure patients with high-risk or recurrent acute lymphoblastic leukemia (ALL). Those lacking a related donor can receive either autologous or histocompatible unrelated donor (URD) marrow. Autotransplantation may result in higher risk of relapse, whereas URD allografts, although associated with serious posttransplant toxicities, may reduce relapse risk. Six years (1987 to 1993) of consecutive autologous BMT (University of Minnesota, Dana Farber Cancer Institute; n = 214) were compared with URD transplants (National Marrow Donor Program; n = 337). Most transplants (70% autologous, 48% URD) were in early remission (first or second complete remission [CR1 or CR2]); 376 patients (75% autologous, 64% URD) were less than 18 years old. Autologous BMT led to significantly lower transplant-related mortality (TRM; relative risk [RR] 0.35; P = .001). URD transplantation offered greater protection against relapse (autologous RR 3.1; P = .001). Patients greater than 18 years old, women, and BMT recipients beyond CR2 had higher TRM, whereas adults, BMT recipients in CR2+, or BMT recipients during 1991 through 1993 had significantly more relapse. After 25 months median follow-up, 100 URD and 56 autologous recipients survive leukemia free. URD BMT in CR2 resulted in superior disease-free survival (DFS), especially for adult patients. Multivariate analysis showed superior DFS for children, men, and BMT during CR1 or 2. Autologous and URD BMT can extend survival for a minority of patients unlikely to he cured by chemotherapy, and the results with either technique are comparable. Greater toxicity and TRM after URD BMT are counterbalanced by better protection against relapse. Prospective studies addressing additional clinical variables are needed to guide clinical decision making about transplant choices for patients with ALL.