Six cases (5 male, 1 female, 17-50 years of age) of delayed radiation necrosis of the brain are presented. Associated condition of these cases were 4 pituitary adenomas, 1 acromegaly and 1 mid-brain tumor. Each case received ⁶⁰Co-irradiation therapy ranging from 4, 850 to 6, 000 rads in tumor dose. Increased intracranial pressure due to space occupying lesion was evident in all cases after 1 to 6 years following radiotherapy. These included headache, vomiting, loss of consciousness, and hemiplegia. The cases were divided into the following two groups by length of latent periods after radiotherapy; one group whose symptoms occurred after 1 to 2 years, and the other group whose symptoms occurred after 4 to 6 years, respectively. Surgical excision of the mass was performed in all cases. Histopathologically, the former group was characterized by coagulation necrosis of the brain tissue while the latter group was characterized by macroscopically evident cyst formation containing bloody fluid. After surgery the symptoms improved, but some neurological deficits remained in the former group. On the contrary, the symptoms were completely eradicated in the latter group. We concluded that the surgical excision of the necrotic or cystic mass was useful for treatment of radiation necrosis.