A study has been made of all admissions to mental hospitals in England and Wales in 1971 by place of birth. After age/sex standardization of rates and reallocating those patients for whom place of birth was not recorded to appropriate categories, several surprising findings emerged. Compared to the native born, those born in Ireland and Scotland had very high rates of mental hospital admission. Poles also had high rates but West Indians and those born in the U. S. A. had rates comparable to the native born. Rates of mental hospital admission for immigrants from India, Pakistan, Germany and Italy were much lower than native born rates. All immigrant groups studied had higher rates of admission for schizophrenia than natives but much of this discrepancy can be explained by the age structures of the populations being compared. The Irish and Scots had extremely high rates of alcohol and drug related disorders and personality and behaviour disorders but West Indians were underrepresented in these diagnostic categories. Various explanations for these and other results are considered and it is concluded that the most tenable hypothesis is one of differential selection for migration — where migration is relatively easy the less stable members of a population self select for migration but where migration is relatively difficult only the most stable individuals can achieve migration.