The decisive conclusions to be drawn from the available epidemiological data, mostly geography and prevalence, of MS are: (1) a north-south (as well as west-east in the United States) gradient exists independent of genetic/racial factors; (2) major differences in prevalence occur in the absence of latitude differences; (3) individuals from the same ethnic derivation have either similar prevalence rates or very different prevalence rates in widely separated geographical areas and (4) specific resistant isolates are shown to exist regardless of latitude. Existing information leads to the almost inescapable conclusion that the epidemiology of MS cannot be explained by any single known environmental or genetic factor(s) in isolation. A combination of a heterogeneous distribution of both genetic and environmental factors appears to be required to explain the available data on MS.