Multiple sclerosis in Finland: evidence of increasing frequency and uneven geographic distribution.

For the present incidence study of multiple sclerosis (MS) in Finland, reliable data since 1964 were available. The mean annual incidence rate of MS was significantly higher in the western part (3.3 per 100,000) than in the southern part of the country (2.2 per 100,000). The mean annual incidence for all MS patients was highest during the second of the three 5-year periods from 1964 to 1978. However, the incidence for women in the western part increased also thereafter. At the same time, the female-to-male incidence ratio changed from 1.0 to 2.2. The results confirmed an uneven distribution with a stable cluster of MS in the western part of the country.