In a national prevalence study of multiple sclerosis (M.S.) in the Republic of South Africa based on census day 1960 there were 118 individuals with M.S. who were born in Northern Europe (United Kingdom and other parts of North and Central Europe) and who had emigrated to the Republic by 1960. Their prevalence rate was 49 per 100,000 immigrants in comparison with a prevalence of 11 per 100,000 among native-born English-speaking white South Africans. To study the possible effect of age at immigration it was necessary to relate the M.S. immigrants to the appropriate denominator—the population at risk according to age at immigration. The population at risk by age at immigration has been estimated by two methods in an indirect fashion with the assistance of the Bureau of Census (1960) and by surveys of the population at risk 1968-9. Both studies suggest that the risk of developing M.S. was reduced to less than a third of the expected risk among those who immigrated under the age of 15 or 16. This study is further evidence that M.S. is an acquired exogenous disease, the precise nature of which is still not certain but, according to present knowledge, has as its leading contender the class of slow, latent, or temperate viruses.