We prospectively studied 60 patients with uncomplicated optic neuritis (ON) to determine the risk of subsequent multiple sclerosis (MS). All patients were followed for at least 5 years (mean, 7.1 years). Seventeen patients (28 percent) developed definite MS and four (7 percent) developed probable or possible MS. Six of the 17 patients who developed definite MS did so within the first year. Forty-five percent of the women but only 11 percent of the men developed MS. Both sexes were at highest risk if the ON occurred between the ages of 21 and 40. Fifty-one percent of patients in this age group progressed to MS, whereas the risk for others was 12 percent. There was an overall increased risk of MS with recurrent ON. The course of the MS appeared to be benign during the period of observation.