Visual evoked potentials (VEPs) were compared among six groups of patients tested at various times after an episode of acute unilateral optic neuritis (ON). The incidence of abnormalities ranged from above 90% in patients tested during the first 6 months to about 70% when more than 2 years had elapsed. Compared with the acute stage (1-8 weeks), latency prolongation was almost 50% less in patients tested after 2-19 years. In the latter group, latencies were significantly correlated with the patients' age at the time of the attack. The findings confirm and extend the evidence of serial studies which shows that the shortening of VEP latency is a general phenomenon which proceeds for up to 2 years and possibly for longer in younger patients. When the patients with clinically isolated ON were compared with those who had a history of additional neurological episodes suggestive of multiple sclerosis (MS), up to 4 weeks after ON latencies were more prolonged in the MS group but between 4 and 8 weeks amplitudes were larger and between 8 weeks and 2 years latencies were significantly shorter. No significant differences were found in patients tested after more than 2 years. It is suggested that the electrophysiological deficit may initially be more severe in patients with disseminated disease, but that recovery may occur faster.