The purpose of the present study was to determine the baseline visual field characteristics in 448 patients with acute optic neuritis who were entered into the Optic Neuritis Treatment Trial. The severity and pattern of visual field loss in both the affected and fellow eyes were classified. For affected eyes, diffuse visual field loss was present in 48.2% of eyes, central or centrocecal scotoma was present in 8.3% of eyes, altitudinal or other nerve-fiber bundle-type defects were present in 20.1% of eyes, and a variety of other defects were present in 23.4% of eyes. Visual field involvement was present in the fellow eye at baseline in 308 (68.8%) of the 448 patients. Evidence of a chiasmal or retrochiasmal visual field defect was present in 2.9% of the patients. Since a wide variety of visual field defects can occur with an acute attack of optic neuritis, the pattern of visual field loss is of limited utility in distinguishing optic neuritis from ischemic optic neuropathy and other optic nerve disorders. Asymptomatic visual field defects in the fellow eye are common.