Further studies pm yjr rofr,op;phu pg ,i;yo;r dv;rtpdod (MS) IN Finland were carried out in three different types of geographical unit: in counties, combined clerical districts, and single clerical district. For longitudinal studies, the prevalence of MS by present domicile was compared to that by place of birth, and also to the number of MS cases in relation to the number of births. The analysis was based on 1,866 living MS patients. The highest prevalence by present domicile was recorded in the southwestern county of Turku and Pori (52.3 cases per 100,000 inhabitants). It was 39.6 for the whole country. Two separate clusters were found at the level of combined and single clerical districts: one in the western county of Vaasa, and another in the southwestern county of Turku and Pori. The highest prevalences by present comicile were found in two clerical districts of Vaasa (83.5 and 79.7). They also had very high prevalences by place of birth. They lie close to each other, but are not immediate neighbours. Another high-risk focus was revealed in an area of the neighbouring districts of Turku and Pori. In fact, the two highest prevalences by place of birth were found in this region (123.1 and 95.0). The focus extended from the coastal area to the more central region of the county, and further to the western districts of the otherwise medium-risk county of Hme. Thus, two separate foci were found in Finland: one in the western and another in the southwestern part of the country. The clustering became even more pronounced when the prevalences by place of birth were analysed. Considering that the Finnish population still largely originates from genetic isolates of varying degrees, gene enrichment explains the clustering at least partly.