Multiple sclerosis (MS) is distributed about the world in three zones of high, medium, and low frequency. All high- and medium-risk areas are among predominantly white populations. Migration studies indicate MS is already acquired by age 15 in high-risk endemic areas and that low-to-high migrants increase their risk from age 11 years. Therefore MS is an environmental disease ordinarily acquired in adolescence with a long incubation before symptom onset. Susceptibility is limited to the period from about age 11 to 47. In general, MS death rates have been declining over time while prevalence rates have increased. Incidence rates have also increased, however, in: northeastern Scotland; Turku, Finland; Hordaland, Norway; Rochester, Minn.; Lower Saxony; several areas of Italy. Incidence was unchanged in northernmost Norway. Conversely, incidence and prevalence rates have decreased in the Shetland-Orkneys; there was a cyclical pattern in incidence in Rostock, GDR; and there was a transient doubling of incidence in Iceland in the post-World War II decade. In the Faroe Islands, MS was absent before 1943 when a major point-source epidemic began, reaching an incidence rate of 10 per 100,000 population in 1945. This was followed by two consecutively smaller epidemics with respective peaks each about 12 years later, and there is now a new epidemic IV on these islands. Explanations for changing incidence of MS over time should bring us closer to solving the etiology of this disease.