Seventeen children with CNS leukemia treated with chemotherapy and 5 children treated with both cranial radiation (CRT) and chemotherapy were evaluated. Eighty-eight percent of patients treated with chemotherapy alone had CT abnormalities, and all treated with CRT and chemotherapy had abnormal CT. The severity of CT abnormality paralleled intraventricular methotrexate levels and clinical signs of leukoencephalopathy. Children who receive chemotherapy for CNS leukemia, even without cranial irradiation, are more likely to have leukoencephalopathy than children without CNS leukemia. Moreover, patients with CNS leukemia may have abnormalities of CSF clearance of intraventricularly administered drugs.