We have previously found evidence for linkage as well as allelic and haplotype association between the myelin basic protein (MBP) gene and multiple sclerosis (MS). These findings have, however, not been reproduced in other populations. Here, we have analyzed association between MBP and MS in a new set of 349 Finnish triad families. Families with a parent born in the Southern Ostrobothnian region in western Finland (Bothnia families, n=98) were analyzed as a separate group since our previous studies included a high proportion of patients and families from this high-incidence region. Other families (n=251) were collected at five hospitals in southern, eastern, and northern Finland. The MBP short tandem repeat was genotyped, and haplotype patterns were verified by sequencing. In the Bothnia families, the previously detected associations with the 1.27 kb allele and haplotype 1.27-B10 were confirmed (P=0.01 and 0.02, respectively), whereas in the other families there was not even a trend toward association. These results demonstrate a geographic/genealogical restriction in the association between MS and the MBP short tandem repeat, highlight the importance of genealogical information in genetic studies of complex traits, and may provide an explanation why the association has not been found in many other populations.