Clinico-pathologic characteristics of radiation-induced brain atrophy were reviewed. Radiation-induced brain atrophy becomes obvious 2 to 3 months after radiation therapy in more than half of the patients who have been irradiated to the brain. Mental and neurologic deterioration were well correlated with the severeness of brain atrophy. Pathologically, demyelinating process secondary to direct neurotoxicity of irradiation seemed to be responsible for the entity. A large target field or whole brain irradiation and the senility of patients were major risk factors. The most important differential diagnosis is normal pressure hydrocephalus which is basically curable. The prognosis of radiation-induced brain atrophy is poor. The only possible method to protect the brain from this pathologic entity is to minimize target field as much as possible.