We evaluated event-free survival (EFS) and leukemia-free interval (LFI) of children treated for acute lymphoblastic leukemia (ALL). Patients were randomized to receive either a low dose or high dose of methotrexate (MTX) as a single agent at the time of diagnosis. Five days later, multidrug therapy was begun. We assessed the early antileukemic efficacy of the two doses of MTX, as well as toxicity and long-term efficacy. An increase in cell kill, as indicated by a larger decrease in the percentage of viable cells in the bone marrow between days 0 and 5, was observed for the high-dose MTX group when compared with the low-dose MTX group (P = .04). At 7.1 years of median follow- up, the 38 children randomized to receive high-dose MTX had a better EFS and LFI compared with the 39 patients randomized to receive low- dose MTX. The 7-year percentages (+/- SE) for EFS were 82% +/- 6% for high-dose MTX and 69% +/- 7% for low-dose MTX (P = .13). The 7-year percentages for LFI were 91% +/- 5% and 69% +/- 7%, respectively (P = .01). We recommend that high-dose MTX be considered as an effective addition to induction therapy in childhood ALL.