PURPOSE: We evaluated the imaging abnormalities of the brain observed during and after treatment of acute childhood lymphoblastic leukemia. MATERIALS AND METHODS: The study group consisted of 30patients (male: female= 19:11; mean age, 64months) with acute childhood lymphoblastic leukemia during the previous ten-year period who had undergone pro-phylaxis of the central nervous system. Irrespective of the CNS symptoms, baseline study of the brain involving CT and follow-up CT or MRI was undertaken more than once. We retrospectively evaluated the imaging findings, methods of treatment, associated CNS symptoms, and the interval between diagnosis and the time at which brain abnormalities were revealed by imaging studies. RESULTS: In 15 (50% ; male : female=9:6 ; mean age, 77months) of 30 patients, brain abnormalities that included brain atrophy (n=9), cerebral infarctions (n=4), intracranial hemorrhage (n=1), mineralizing microangiopathy (n=2), and periventricular leukomalacia (n=3) were seen on follow-up CT or MR images. In four of nine patients with brain atrophy, imaging abnormalities such as periventricular leukomalacia(n=2), infarction (n=1) and microangiopathy (n=1) were demonstrated. Fourteen of the 15 patients underwent similar treatment; the one excluded had leukemic cells in the CSF. Six patients had CNS symptoms. In the 15 patients with abnormal brain imaging findings, the interval between diagnosis and the demonstration of brain abnormalities was between one month and four years. After the cessation of treatment, imaging abnormalities remained in all patients except one with brain atrophy. CONCLUSION: Various imaging abnormalities of the brain may be seen during and after the treatment of acute childhood lymphoblastic leukemia and persist for a long time. In children with this condition, the assessment of brain abnormalities requires follow-up study of the brain.