Forty remission patients with high-risk acute lymphoblastic leukemia (ALL) underwent matched allogeneic bone marrow transplantation (BMT) following preparation with cyclophosphamide and fractionated total body irradation (TBI). As of March 1987, the median follow-up is more than 3 1/2 years. Thirteen patient are alive (11 relapse free) between 2 and 4 1/2 years post-BMT. Neither age, sex, remission number, prior extramedullary leukemia, nor WBC at diagnosis of ALL was statistically significant as a predictor of relapse-free survival. The development of acute graft-versus-host disease (GVHD) in 17 patients was found, with time-dependent Cox regression analysis, to be associated with a significant reduction in post-BMT relapse risk (P=.04) and improved disease-free survival (P=.11). A prospective, randomized trial of maintenance chemotherapy with oral methotrexate and mercaptopurine did not demonstrate improvement in relapse risk or survival for those assigned maintenance chemotherapy (P=.7). These results suggest that allogeneic BMT can result in extended relapse-free survival for some patients with high-risk ALL. More effective preparative chemoradiotherapy and exploitation of the apparent graft-v-leukemia effect may be useful in future trials.