With increasing survival statistics in childhood acute lymphoblastic leukemia, long-term sequelae of therapy have become apparent. Prophylactic treatment of the CNS has significantly contributed to this improved survival. CNS prophylaxis has been done in a variety of ways. Cranial radiation therapy and intrathecal medication are used in some regimens, whereas intrathecal medicines plus systemic chemotherapy are used in others. The acute and late effects of cranial radiation are now being assessed by long-term follow-up of survivors of acute lymphoblastic leukemia. Neuropsychiatnic testing and computed tomography (CT) evaluation have been the parameters used in testing. CT findings, even as early as several months after irradiation, have included ventricular dilation, widening of subarachnoid spaces, areas of hypodensity, and intracerebral calcifications.