Thirty asymptomatic patients with acute lymphoblastic leukemia who had received prophylactic cranial irradiation (16 pts had 2400 cGy, 14 pts 1800 cGy) and intrathecal methotrexate were studied by computed tomography of the brain 60 to 148 months after initiation of prophylaxis. Three of 30 (10%) patients presented abnormal findings: widening of frontal subarachnoid space (1 patient), little area of decreased attenuation coefficient (1 patient), and intracerebral calcifications (1 patient Tomography abnormalities could be detected either in patients treated with 2400 cGy and in those treated with 1800 cGy. None of our patients showed central nervous system dysfunctions on physical examination. The results of our study suggest that tomography findings have a poor clinical significance.