Although the cause of multiple sclerosis (MS) is unknown, epidemiologic studies support both genetic and environmental components of susceptibility. Reports of clusters, small "epidemics," geographic variation in prevalence, and alteration of MS susceptibility by migration support an environmental factor (or factors). The higher risk for MS in Europeans and in relatives of patients and the existence of MS-resistant ethnic groups support genetic predisposition. Findings from epidemiologic studies are most consistent with an environmental influence occurring on a background of genetic susceptibility as the cause of MS. Currently, there is little evidence for a single or unique environmental cause of MS.