An incidence study of idiopathic optic neuritis (ON) was carried out in 2 geographic areas of Finland for the 9-year period 1970 to 1978. The southern province of Uusimaa composes a medium-risk and the western province of Vaasa a high-risk area for multiple sclerosis (MS). The risk for subsequent MS was determined. A total of 315 attacks on ON were recorded in 296 patients. The mean annual age-adjusted incidence for ON in Uusimaa was 2.2 and in Vaasa 2.5 per 100,000 population. The incidence figures remained unchanged all the time. The mean age at onset was 30 years. 19% of ON patients developed MS during the mean follow-up period of 5.1 years. When the life-table method of analysis was used, the probability of developing MS was 38% in Uusimaa and 24% in Vaasa 9 years after acute optic neuritis. In Uusimaa the risk of women for MS was significantly higher than in men. In 47%, the MS symptoms and signs developed within 1 year and in 90% within 5 years after the initial bout of ON. It is possible that only one part of idiopathic ON cases do have a relationship with MS.