Thirty-eight patients with poor risk ALL in first remission received maintenance chemotherapy following ABMT. Patients were conditioned for ABMT with high-dose melphalan and single fraction total body irradiation. Maintenance chemotherapy was commenced in a total of 26 patients and was tolerated to a median daily dose of 6-mercaptopurine of 40.5 mg/m2 and a median weekly dose of MTX 8.3 mg/m2. Twenty patients remain alive in first remission with a projected disease-free survival of 50% and a median follow-up in survivors of 200 weeks (range 48-387 weeks). Eleven patients have relapsed at a median of 4.5 months from ABMT. These patients were compared with remission patients with ALL receiving conventional chemotherapy on the United Kingdom Medical Research Council trials UKALL X and XA. After stratifying for major risk factors and allowing for the delay from remission to transplant, we have shown a significant reduction in the risk of relapse after ABMT (p = 0.04). Disease-free survival was not significantly increased due to transplant-related toxicity. This study suggests that maintenance chemotherapy to prevent relapse after ABMT for ALL is well tolerated and warrants assessment in a formal controlled trial.