The Faroe Islands are a semi-independent unit of the Kingdom of Denmark and are located in the North Atlantic Ocean between Norway and Iceland. Efforts to identify all cases of multiple sclerosis (MS) since 1900 among Faroese have been continuing for over a quarter century. As of 1998 prevalence was 66 per 100,000, age adjusted to 1960 US population, with a rate of 100 for women and 34 for men. Median survival was at 29 to 34 years with no significant difference by sex. Faroese with overseas residence indicated that at least 2 years of exposure from age 11 on in a high-risk area are required for acquisition of MS. Among native resident Faroese the first instance of symptom onset was in 1943, heralding a type 1 epidemic of 21 cases. This was followed by three successive epidemics of 10, 10, 13 cases, with membership in each epidemic defined by calendar time and age of exposure. Age at exposure for epidemic I was 11 to 45 years; for later epidemics age 11 was the minimum. We believe the source of MS on the Faroes was their occupation by British troops for 5 years in World War II. We think they introduced a widespread, specific, persistent (but unknown) infection, probably asymptomatic, which we call the primary multiple sclerosis affection (PMSA). Only a small proportion of those affected with PMSA will years later show any clinical signs of MS. Models of transmission of PMSA through successive cohorts of Faroese fit the data for epidemics II and III, and predicted the occurrence of epidemic IV. The Faroese provide an ideal location to determine the nature of PMSA, since the disease has remained geographically stable for 50 years without further spread throughout the islands.