This research was carried out under the aegis of the Committee on Migration Differentials. It is a study of first admissions to psychiatric hospitals in New York during 1939–1941. The results are very striking indeed: during that period, admissions were markedly higher for migrants than for non-migrants, regardless of sex and colour. This was true for all psychoses including those of old age. The differential was even more marked for recent than for earlier migrants. The authors warn against rash interpretations. Although their data suggest that rates of mental illness are higher for migrants than for non-migrants, a number of reservations have to be made before drawing such a conclusion. Admission to hospital is not a reliable index of the incidence of mental illness among a special section of a population. This study is a model of its kind, especially through the absence of special pleading in favour of a particular etiological factor.