Clustering of multiple sclerosis in the county of Hordaland, Western Norway

We studied the distribution of MS cases in the county of Hordaland, Western Norway. The total MS population comprised 426 patients. The prevalence on January 1st, 1963, and on January 1st, 1983, and the average annual incidence in the period 1963-1982 were all lower in the coastal area compared to the inland area. An increase in incidence started in the urban area and was followed some 10 years later in the rural area. The Knox method revealed no statistically significant evidence of clustering either for time/place of onset or for time/place of birth. Indications of clustering according to year of onset were, however, observed in the rural area.