Recurrent symptoms after radiation therapy of a brain tumor are not always the result of regrowth of the neoplasm but may represent radiation necrosis of the brain. Seventeen patients with proven cerebral radionecrosis are included in the present study. In five patients who received 5,000 to 5,500 rads, areas of radiation necrosis of the brain were shown on computed tomography (CT) as low density lesions with no mass effect or contrast enhancement. In 12 patients who received 6,000 to 7,000 rads, radionecrosis was shown on the CT scans either as a localized mass with contrast enhancement or as diffuse low density, isodense, or high density lesions with no mass effect but with occasional enhancement. In all cases, the lesions coincided well with volumes of the brain that received a threshold dose of radiation determined from the isodose reconstruction maps.