The baseline characteristics of 448 eligible patients entered into the Optic Neuritis Treatment Trial are described in an effort to summarize the clinical profile of acute optic neuritis. A total of 77.2% of the patients were women. Mean age was 31.8 years. Pain accompanied the visual loss in 92.2% of cases. The optic disc appeared swollen in 35.3% of the patients and normal in 64.7%. A wide variety of visual field defects were present. Abnormalities in asymptomatic fellow eyes were noted, particularly on perimetry. Magnetic resonance imaging showed changes consistent with demyelination of the brain in 48.7% of the patients. Magnetic resonance imaging, serologic studies (such as the antinuclear antibody test and the fluorescent treponemal antibody absorption test), chest roentgenography, and lumbar puncture were of limited utility in defining a cause for visual loss other than optic neuritis associated with demyelinative disease.