The demographic and clinical characteristics of 89 multiplex families whose affected members meet proposed diagnostic criteria for multiple sclerosis (MS) genetic research are described and compared with 425 sporadic cases of MS and other published collections of MS multiplex families. The proportion of affected multiplex family members who experienced gradual progression of disability from onset (primary progressive MS) is lower than reported by other investigators. Different phenotypes of MS may reflect genetic heterogeneity that may partially explain inconsistencies in the results of genetic linkage studies. Clinical details of affected multiplex family members must be described so that comparisons of genetic results across studies can be properly interpreted.