We studied 88 patients with isolated idiopathic optic neuritis (IION) in order to evaluate the rate of progression to multiple sclerosis (MS) in Brazil. The patients were reassessed from one month to nine years after the development of the HON (mean follow-up was 4.6 years). There were 52 men and 36 women with ages ranging from three to 59 years (mean 24.3 years). Bilateral optic neuritis occurred in 19 patients whereas sequential involvement of the fellow eye after an interval longer than four weeks occurred in other 19 patients. Recurrences in the same eye occurred in seven cases. Nine patients (10.8%) developed clinically definitive MS - 13.9% of the women and 7.7% of the men with IION. The median age at the time of diagnosis of MS was 25 years. The mean interval between HON and the emergence of other MS signs varied from one month to five years - median one year. Sixty-seven percent of these, patients developed signs of spinal cord involvement. Our findings when compared to published series in different countries are closer to figures reported in Japan than those in the West.