Four hundred and eighty-seven patients undergoing allogeneic or autologous BMT for acute leukemia in first remission received no prophylactic intrathecal chemotherapy after BMT. The conditioning regimen included total body irradiation in 433 (89%). Patients with acute lymphoblastic leukemia received cranial irradiation if they had no central nervous system (CNS) disease and all patients with CNS disease received craniospinal irradiation. Eleven of 311 patients examined had CNS disease at presentation, but none had active CNS disease at the time of BMT. Lumbar punctures were performed in 93 patients 1-2229 days (median 98) after BMT because of suspected CNS infection, hemorrhage or relapse (n = 65), or after systemic relapse (n = 28). CNS disease was seen in seven patients at 217-1209 days (median 340), none of whom had CNS disease pre-transplant. Systemic relapse had preceded CNS relapse by 4-207 (median 128) days in all seven and no isolated CNS relapse was seen. The actuarial risk of CNS and systemic relapse at 5 years was 2.9 and 35.9%, respectively. We conclude that CNS relapse is uncommon after BMT for acute leukemia in first remission, and that isolated CNS relapse is likely to be extremely uncommon. Therefore, routine prophylactic intrathecal chemotherapy is not warranted after BMT for acute leukemia in first remission.