Nine patients with a history of radiation of 2400-6000 rad (24-60 Gy) to the brain were examined by magnetic resonance imaging (MRI) and computed tomography (CT). MRI demonstrated abnormalities in the periventricular white matter in all patients. The abnormal periventricular signal was characterized by a long T2 and was demonstrated best on coronal spin-echo (SE) 1000/80 images. A characteristic scalloped appearance at the junction of the gray-white matter was seen on MR images of seven patients, and represented extensive white-matter damage involving the more peripheral arcuate fiber systems. This differs from transependymal absorption, which is seen best on SE 3000/80 images and has a smooth peripheral margin. Cranial CT demonstrated white-matter lucencies in six cases but generally failed to display the extent of white-matter injury demonstrated by MRI. MRI is uniquely suited to detect radiation injury to the brain because of its extreme sensitivity to white-matter edema.