Eight of 16 patients with isolated idiopathic optic neuritis were found to have one to several brain lesions by nuclear magnetic resonance scanning. The brain lesions were predominantly located in the periventricular white matter; their appearances, locations, and extents were similar to those seen in recent nuclear magnetic resonance studies of patients with definite multiple sclerosis. All of these brain lesions were clinically silent and were missed by computerized tomography. Idiopathic optic neuritis may be the only manifestation of a multicentric disease process that is disseminated in the central nervous system in the majority of cases.