Granulomatous angiitis of the brain (GAB) is defined histologically by granulomatous inflammation of intracranial blood vessels. We have studied four patients with autopsy-proved GAB who had, respectively, Hodgkin's lymphoma, herpes zoster, neurosarcoidosis, and no associated illness. Headache, fever, or mental change was followed by hemiparesis or quadriparesis, coma, and death in all four patients. There were no diagnostic findings from cerebral computed tomograms, cerebrospinal fluid, or cerebral angiograms; the diagnosis was established only by postmortem examination. Vasculitis was limited to the brain in all four patients, and involved large arteries, small arteries and veins, or both large and small vessels. Differences in etiology and different particulars of the pathologic conditions imply that GAB is a nonspecific reaction, not a unique disease. The diagnosis, moreover, cannot be proved without histologic confirmation. A biopsy specimen is the only way to ascertain the diagnosis in living patients.