A study of multiple sclerosis (MS) frequency among immigrants to Israel revealed that those from Afro-Asian countries, where MS is rare, had a low prevalence of MS, while European immigrants had a relatively high prevalence of the disease. On the other hand, native-born Israelis had a high rate of the disease, like European immigrants, regardless of the region of origin of their forebears. Age-specific incidence rates plotted for different age-at-immigration cohorts and based only on the most rigidly defined cases suggested that Afro-Asians who immigrated very early in life had rates similar to European immigrants. Among immigrants who came to Israel at an older age, Europeans had higher age-specific incidence rates of MS than Afro-Asians. Although the number of patients available for analysis in the youngest age-at-immigration cohort was small, and the age groups for which specific incidence rates could be plotted were still few, the results indicated that migration modifies risk only among those who migrate early, before adolescence. Over the next decade, as this immigration cohort ages, the validity of this result can be verified. If correct, it would mean that MS can be acquired in the brief interval between infancy and preadolescence.