Visual function in the fellow eye at the onset of unilateral optic neuritis has not been systematically evaluated. The authors prospectively determined the prevalence of abnormalities in the fellow eyes of the 448 eligible patients entered into the Optic Neuritis Treatment Trial.All patients underwent testing of visual acuity, contrast sensitivity, color vision, and visual field, as well as magnetic resonance imaging (MRI) of the brain and a neurologic examination.Abnormalities in the fellow eye were found on measurement of visual acuity in 13.8%, contrast sensitivity in 15.4%, color vision in 21.7%, and visual field in 48.0% of patients. The majority of the fellow eye deficits resolved over several months. A higher prevalence of MRI changes consistent with demyelination of the brain was found in patients with a past history of optic neuritis in the fellow eye compared with patients without such a history (P = 0.004). Patients with abnormal fellow eyes but no history of previous optic neuritis were no more likely to have clinical (P = 0.658) or MRI evidence (P = 0.166) of multiple sclerosis than patients with normal fellow eyes.The improvement of many of the visual deficits indicates that visual abnormalities detected in the fellow eye at the onset of symptomatically unilateral optic neuritis may not represent preexisting optic nerve demyelination. Whether the presence of these deficits is predictive of the development of clinical multiple sclerosis cannot be determined at this time.