The epidemiology of multiple sclerosis (MS) was investigated in a small selected area within the western high-risk county of Finland. The investigation was extended to the level of single communes, villages and even houses, together with the search for all familial cases born in this district. The results were compared to those obtained for Helsinki, a city of medium-risk for MS. Prevalences by present domicile that exceeded 100 per 1000,000 inhabitants were recorded in several communes of the western high-risk county. The highest prevalence was 174.2. Seventy patients were born in the small high-risk area. This was 25% of the MS patients born in the whole county and much higher than expected (16%). A positive familial history of another MS patient was recorded in 8 cases (11%). They were all living, first-degree relatives. A similar history was found in only 2 cases (2%) among the 99 MS patients born in Helsinki. The birthplaces of the 123 parents of these 70 MS cases could be confirmed. All villages with high MS frequencies were located along the rivers, running through the area. The birthplaces of the patients showed a similar accumulation to the valleys. No conjugal cases were found. If the preponderance of familial cases in the small high-risk area reflects the role of genetic factors in the aetiology of MS, it is only of polygenic nature. The pronounced clustering of the birthplaces in the small high-risk area and, especially, along the rivers also suggests the importance of environmental influences in early childhood.