One hundred and four patients treated with radiotherapy for intracranial tumors and their related conditions were reviewed to evaluate the usefulness of magnetic resonance (MR) imaging in demonstrating increased signal intensity areas on T2-weighted images that were considered to be late adverse effects of irradiation of the brain. High signal intensity areas of the white matter were divided into five patterns according to their size and extension. Severity was found to increase with age and irradiation doses of more than 50 Gy. In patients with irradiation doses of more than 60 Gy, the severity of increased with shorter interval after radiotherapy than in those given low irradiation doses. Clinical findings such as mental deterioration, motor abnormality, and visual defect were observed in 12 patients. These findings were closely correlated with the severity of the MR pattern. In most patients, high signal intensity areas were stable or progressive during the course of follow-up. However, these areas were regressive in three patients. Imaging with Gd-DTPA was performed in 36 patients, six of whom showed enhancement. Pathological findings on enhancement included astrocyte proliferation and coalescing vacuoles in neural tissue. MR imaging is an excellent method with which to monitor the adverse effects of radiotherapy of the brain.