Previous surveys in Finland from the 1960s have documented an uneven geographic distribution of multiple sclerosis (MS). In the present study, the incidence of MS was studied during 1979-1993 in the western Vaasa and Seinäjoki regions and in southern Uusimaa. The overall difference between the western and southern regions persisted; 8.7 per 100,000 in the western, and 5.1 per 100,000 in the southern region. The incidence of 11.6 per 100,000 in Seinäjoki was more than twofold greater than the 5.2 per 100,000 incidence found in neighboring Vaasa. An increasing incidence trend was observed for men in Seinäjoki, and a decrease for both sexes in Vaasa, while in Uusimaa the incidence remained stable for both sexes. The different incidence trends could not be readily explained by differences in case ascertainment but suggest the effect of environmental factors that have modulated the incidence of MS during the 15-year study period.