We have investigated whether any prognostic factor can be used to identify those children who have a relapse after discontinuation of therapy for acute lymphoblastic leukemia (ALL). Our population-based series comprised 167 children with newly diagnosed ALL. The 3-year event-free survival rate in these children was 65%. Maintenance therapy was electively discontinued for 120 patients, 20 of whom have subsequently had a relapse 1 to 27 months later. In multivariate analysis the risk of late relapse in the 15 patients with initially enlarged kidneys was 4.5-fold (95% confidence limits 1.7-11.8) that of the others (p less than 0.01). The risk in the 18 patients with initially elevated CSF protein concentration (greater than 0.4 g/l) or leukocyte count (greater than 5 x 10(6)/l), but with no blasts in the CSF, was 3.8-fold (1.5-9.6) that of the others (p less than 0.01). Our results indicate that enlarged kidneys or abnormal CSF findings at initial diagnosis are associated with an increased risk of late relapse in children with ALL.