To re-evaluate how migrant studies contribute to our understanding of the epidemiology and aetiology of multiple sclerosis, we undertook a systematic review of all relevant studies published in English language journals, comparing rates of multiple sclerosis in migrant populations with those in the host country and in the country of origin. Interpretation of migrant studies is difficult. Migrants are seldom representative of the country of origin, tending to be younger, healthier and of higher socioeconomic status. Data quality may be poor, and lack of age-standardisation can mislead when rates are compared. Nevertheless, two consistent patterns can be discerned in these studies of the effect of migration on risk of multiple sclerosis. Migrants who move from an area where the disease is common to an area where it is rarer show a decrease in rate of disease. By contrast, people who migrate in the opposite direction tend to retain the low risk of their country of origin. Results from the few studies which have examined the effect of age at migration on risk of multiple sclerosis suggest that an individual's risk of the disease is largely established during the first two decades of life. Risk of multiple sclerosis can change rapidly between generations: although migrants from low risk countries to high risk countries retain their low risk, their children have a risk of multiple sclerosis that approaches that of the host country. Migrant studies add little to our understanding of the genetics of multiple sclerosis but they emphasise the importance of environmental factors. We discuss several possible interpretations of the patterns seen in migrant studies, including the hypothesis that multiple sclerosis is a sequel to delayed exposure to a common infectious agent. One candidate for such an infectious agent is Epstein-Barr virus.