Multiple sclerosis (MS) prevalence was ascertained among the 7 million residents of Los Angeles County, California, and the 1.5 million residents of King and Pierce Counties In Washington state. The relationship between age at immigration and the prevalence of MS In migrants to these two areas from within the United States was examined. Groups migrating from the North had the highest prevalences, although those migrating to Los Angeles had a lower prevalence than those to King-Pierce Counties. MS prevalence was higher In successively older age-at-immigration groups regardless of place from which or to which they migrated. Differences In prevalences between Los Angeles and King-Pierce residents from the North were greatest in early migrants (<10 years) and lowest In later migrants (15–19 years). Within age-at-immigration groups from the South to either area the prevalences were similar. The results suggest that events which precede onset by years to decades are important In determining probability of developing MS although outcome can still be modified, at least In some individuals, after age 15 years, by migration to a lower risk area; that a protective mechanism occurs at a higher frequency In the South; that the causative mechanism occurs more frequently in the North, although it Is uncommon in any area; and that there Is an “age-at-migration effect” manifested by a positive association between prevalence of MS and age at migration.