Five hundred four children with low risk acute lymphocytic leukemia (previously untreated, age 3 to 6 years with white blood counts less than 10,000/mm3 at diagnosis) were randomized into two different central nervous system prophylaxis regimens. One regimen (250 patients) consisted of cranial radiation and intrathecal methotrexate (IT MTX). The second regimen (254 patients) consisted of IT MTX only. Median follow-up time for surviving patients is currently 54 months from randomization. Life table analysis of central nervous relapse, marrow relapse, disease-free survival, and survival shows very similar outcome for both treatment groups. The results indicate that maintenance IT MTX as described in this report can be substituted for cranial radiation in children with low risk ALL.