Molecular monitoring by quantitative PCR techniques of residual leukaemia cells during the first phases of treatment can predict outcome in children with acute lymphoblastic leukaemia. We did a retrospective study of 30 children who had been treated according to the ALL-REZ BFM trials to assess whether amount of minimal residual disease during the first stages of treatment for relapsed acute lymphoblastic leukaemia could predict outcome. In children with minimal residual disease of less than 10(-3) at day 36, probability of event-free survival was 0.86 (95% CI 0.77-0.95), compared with 0 in children with minimal residual disease of 10(-3) or greater (p<0.001). Our results suggest that information about molecular response to treatment can be used to predict long-term outcome in relapsed childhood acute lymphoblastic leukaemia.