To investigate the pathologic consequences of therapeutic radiation, this morphologic study evaluated the brains of 25 patients with intracranial gliomas treated both with and without this form of therapy. Although beneficial effects such as the retardation of tumor growth were evident in these studies, among the seventeen patients who received from 5000--6000 rads for malignant gliomas, four developed "late delayed" radiation necrosis. The strong predilection of this tissue response for the white matter adjacent to the neoplasm suggests a local sensitivity which may have been engendered or enhanced by cerebral edema. A fifth case disclosed focal demyelination in the mid-brain suggesting "early delayed" radiation necrosis, and an additional case had distinctive foci of necrosis within the brain stem. Changes of diffuse cerebral edema were noted in many of the radiated brains. It is concluded that radiation therapy in commonly employed dosages for malignant gliomas carries a risk of injury to surrounding cerebral tissues.