Among immigrants resident in greater London from Europe, Ireland, the USSR, the old Commonwealth countries of Australia, Canada, and New Zealand, North and South America, Egypt, Turkey, and Iran the incidence of admission to hospital for probable multiple sclerosis (MS) between 1960 and 1972 was high or moderately high. The incidence was the same order as that found in those born in the United Kingdom. Immigrants from India, Pakistan, and other Asian countries and from new Commonwealth Africa and America, which includes the West Indies, had a low incidence of hospital admission for MS. Immigrants from countries where the risk of MS is low whose parents were born in Europe had a reduced incidence of admission to hospital but not the very low incidence found in those parents were also born in these countries. Emigrating to England from low risk parts of the world did not seem to increase the risk of developing MS.