108 children with standard-risk acute lymphocytic leukaemia (ALL) were randomised to a post-induction treatment protocol including 15 doses of intermediate-dose methotrexate (1000 mg/m2) in addition to conventional oral therapy of mercaptopurine and low-dose methotrexate. After median follow-up of 26 months, 22 patients have had relapses. Among the 108 patients, rates of methotrexate systemic clearance ranged from 44·7 to 132 ml/min/m2. When the group was divided into three subgroups according to the patients' rates of methotrexate clearance, statistical analysis of the Kaplan-Meier curves estimating the probability of complete remission showed significant differences (p=0·016) among the subgroups, patients with faster clearance having higher probability of relapse. Multivariate Cox's regression analysis incorporating other potential prognostic variables identified three significant variables influencing the risk of relapse-methotrexate clearance and white-blood-cell count and haemoglobin level at diagnosis (p=0·0015). This study has demonstrated the potential clinical importance of the rate of drug clearance in children with ALL.