Expression of the main classes (pi, mu, and alpha) of glutathione S-transferase (GST) was assessed in the blasts of children presenting with acute lymphoblastic leukemia using an immunohistochemical technique. Bone marrow trephine biopsies obtained at presentation from 71 cases were studied (42 boys, 29 girls; age range, 6 months-14 years; median age, 4 years) and expression was correlated with event-free survival. The period of follow-up was 12-108 months, during which time 21 patients (30%) relapsed. All the samples examined were negative for alpha class GST. Samples from 8 patients, all of whom remained in remission at the time of analysis, were found to be negative for pi class GST at presentation. Samples from 44 (patients were negative for mu class GST (62%); of these, 36 patients (82%) remained in remission. In comparison, of the 27 patients who were positive for mu class GST, only 14 (52%) remained in remission. Analysis of event-free survival demonstrated that expression of mu class GST predicts a 3-fold increased risk of relapse (95% confidence interval, 1.25-7.26). This risk factor appears to be independent of other recognized prognostic factors.