An epidemiological study of the first admission rate for mania was carried out in London and Aarhus. The case registers in these two centres were used to conduct a retrospective study of case notes covering several years, and a screening procedure was used for the prospective collection of new cases over the course of one year. The annual incidence of mania was found to be virtually identical in both centres. The retrospective study gave a figure of 2-6 per 100,000 population in both Aarhus and London. But the London sample was found to contain 45 per cent of immigrants in contrast to the Aarhus sample in which only a negligible proportion were born outside Denmark. Male West Indians, in particular, were over-represented in the London sample.