We report a largely retrospective analysis of minimal residual disease (MRD) in a cohort of 66 children suffering from acute lymphoblastic leukaemia (ALL). All patients lacked high-risk features at diagnosis, i.e. the presenting white cell count was <50 x 10(9)/l, age 1-16 years and translocations t(9;22) and t(4;11) were not present. All were treated according to either the MRC protocols UKALL X or XI. PCR of IgH, TCRdelta and TCRgamma gene rearrangements and allele-specific oligoprobing were employed for the detection of MRD. Sensitivity was at least 10(-4) in 78/82 (93%) probes examined. A total of 33 patients relapsed (seven on therapy and 26 off) and 33 remain in continuing complete remission (CCR) (median follow-up 69 months from diagnosis). Of those who remain in CCR, MRD was present in the bone marrow in 32%, 10% and 0% at 1, 3 and 5 months into therapy respectively. This is in marked contrast to the presence of MRD at these times in 82%, 60% and 41% of patients who relapsed (P<0.001, P<0.005 and P<0.005). These results provide further evidence of a strong correlation between clearance of MRD early in therapy and clinical outcome in childhood ALL.