Between 1972 and 1979, 214 children with acute lymphoblastic leukemia and no evidence of central nervous system (CNS) disease prior to CNS prophylaxis were treated with 2400 rad cranial irradiation and concurrent intrathecal methotrexate. Only nine children developed CNS leukemia; five of them in the CNS only and four concurrently in the CNS and another site. Major acute effects of CNS prophylaxis were seizures in seven patients (3%). Sixty-nine children who had a minimum follow-up of 4 yr were evaluable for late effects of therapy. Small cataracts, incomplete regrowth of hair, and learning disabilities were noted. The latter occurred in 18% of patients, an incidence similar to that encountered in a normal community of school-age children. However, the incidence of learning disabilities in patients who were under 5 yr of age at the time of diagnosis was much higher, 35%. We conclude that the combination of cranial irradiation and intrathecal methotrexate was highly efficacious. The incidence and severity of neuropsychologic abnormalities, the principal late morbidity of this treatment program, varies among reporting institutions. Prospective longitudinal studies of neuropsychologic function are necessary to better define the incidence of abnormalities. Future programs should attempt to decrease late morbidity, but must also assure equal efficacy and improve overall disease-free survival.