Multiple sclerosis (MS) emerged in the French West Indies (Martinique and Guadeloupe) at the beginning of the 1990s with an incidence for the period 1992-2007 of 1.27/100,000 (95% CI, 1.16-1.38). Its incidence, which is continuously increasing, reached 2.27/100,000 (95% CI, 1.92-2.62) in Martinique for the period 2002-07. Neuromyelitis optica (NMO), in contrast, has a stable incidence of 0.19/100,000 (95% CI, 0.15-0.23) resulting in a gradual increase in the MS/NMO ratio, which was 8.35 for the period 2002-07. MS initially affected the female population with a female/male incidence ratio of 8.43 for the period 1992-97, which gradually fell to 2.63 for the period 2002-07 due to the increased incidence of MS in men. Several environmental factors are involved in the emergence of MS: (1) the migration of the French West Indian population to mainland France resulted in an excess incidence of MS in the migrants; (2) a decrease in sun exposure, a risk factor for MS, related to the changing lifestyle of the French West Indian population has been demonstrated in a case-control study; (3) at ecological level, enhanced hygiene associated with the eradication of intestinal helminthiases, known to induce immune system tolerance, is also strongly suspected with regard to MS emergence.