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Among some 7500 respondents with known place of birth who had completed a nationwide questionnaire survey for multiple sclerosis (MS) in France in 1986, there were 260 born in former French North Africa (Algeria, Morocco, Tunisia). They had migrated to France between 1923 and 1986, but 66% came between 1956 and 1964. Two-thirds were from Algeria, where virtually the entire European population had emigrated in 1962 at the end of the Algerian war for independence. The migrants were younger at prevalence day (mean 43.4 years) and at onset (29.4 years) than the French-born MS (46.6; 31.3 years). Eight migrants lacked age information. The 225 migrants with onset more than 1 year after immigration presumably acquired their MS in France. They provided an age adjusted (US 1960) MS prevalence rate 1.54 times that for all France. If the latter is taken at 50 per 100,000 population their estimated adjusted rate is 76.8 with 95% confidence interval of 67.1 to 87.5. The other 27 with presumed acquisition in North Africa gave an estimated adjusted prevalence of 16.6 per 100,000 (95% CI 10.9-24.1). For those migrants with acquisition in France there was a mean interval of 13 years between immigration or age 11 and clinical onset, with a minimum of 3 years. This series provides further support for the theses: 1) that MS is primarily an environmental disease acquired after childhood; 2) that acquisition requires prolonged or repeated exposure (here 3 years for these medium-to-high MS risk migrants) followed by a prolonged latent or incubation period between acquisition and symptom onset (here 10 years); and 3) that this disease is most likely a widespread but unknown persistent infection which results in clinical MS in only a small proportion of those affected.