Since optic neuritis occurs frequently in multiple sclerosis (MS), it has been thought to herald the onset of MS in the young adult where no specific cause can be identified. This study was designed to examine the incidence of optic neuritis in the resident population of Rochester, Minn, in order to clarify the relationship between optic neuritis and MS. Thirteen to fifteen percent of the patients with MS in the Rochester population presented with optic neuritis, and 27% to 37% of the MS patients showed evidence of optic neuritis during the course of their disease. Prospective analysis of idiopathic optic neuritis cases revealed that the chance of its progressing to MS was approximately 17%. The unfavorable prognosis previously associated with idiopathic optic neuritis may be the result of a bias of clinical series. © 1972, American Medical Association. All rights reserved.