We prospectively studied 60 white patients living in New England who presented with uncomplicated optic neuritis (ON) to determine the risk of developing multiple sclerosis (MS). The diagnoses of ON and MS were made solely on the basis of clinical criteria. Patients were followed for a mean of 14.9 years. Life table analysis indicated that 74% of the women and 34% of the men will have developed MS 15 years after their attack of ON. The risk of developing MS was 3.4 times greater for women than for men. Onset of ON between the ages of 21 and 40 years may have had a modest effect in increasing the risk of developing MS. Recurrence of ON did not appear to affect the risk.