Multiple sclerosis is unevenly distributed throughout the world. Its prevalence depends on latitude: it decreases in each hemisphere from pole to equator. France is situated in a high prevalence zone, with 40 cases for 100,000 inhabitants. The prevalence of multiple sclerosis is modulated by risk factors unrelated to latitude; genetic susceptibility factors (HLA, Gm), as well as environmental, occupational, nutritional and infectious (notably viral) factors have been identified, but no conclusion can be drawn concerning their role in the aetiology and pathogenesis of the disease. Among the hypotheses that could put an end to this deadlock, the heterogeneity of multiple sclerosis must seriously be considered, and it might serve as a basis for further epidemiological studies benefiting from recent technological developments such as molecular genetics and magnetic resonance imaging.