The epidemiology of multiple sclerosis (MS) in Finland has been under continuous investigation since 1964. A small rural area in the western part of the country was selected for detailed familial analysis. In this area the prevalence ratio was 101/ 100,000 and the age-adjusted mean annual incidence 4.1/100,000. Of 51 probands, 29% had a second case of MS in their family. In the 51 sibships, 236 sibs were alive and 8% of them were familial cases of MS. The recurrence risk of MS for sibs was 2.5%. This means that MS occurs 25 times more often among sibs of MS patients than in the general population of the same geographic area. Of living sibs, 60% were still at risk for subsequent MS.