OBJECTIVE To investigate the relationship between the clinical outcome of childhood acute lymphoblastic leukemia and the status of minimal residual disease after termination of therapy. METHODS A total of 18 children with acute lymphoblastic leukemia were studied. Eleven cases were treated with chemotherapy, the other 7 with autologous peripheral blood stem cell transplantation (ABSCT) or autologous bone marrow transplantation (ABMT). The outcome of the 18 patients was obtained by clinical follow-up. Minimal residual disease was detected by means of the polymerase chain reaction (PCR) and restriction analysis. RESULTS Minimal residual disease was detected in 9 patients after completion of therapy. Three out of the 9 patients relapsed within 3 to 6 months. Among the other 9 patients with no detectable minimal residual disease, no one relapsed. CONCLUSIONS PCR detection of minimal residual disease after termination of therapy can identify patients at risk for relapse in future. Minimal residual disease that cannot be detected after termination of therapy may be one of the important factors for long-term disease-free survival.