SynopsisCensus data from the Danish island of Samsø have provided information on all but 20% of the population. Fifty per cent of the population with past or present mental illness had been referred to the psychiatric service during the 18-year-period 1957–74; the proportion varied from 85 % for patients with psychoses (100 % for schizophrenic patients, and 90% for manic-depressive patients) to 43 % with non-psychotic disorders. The frequency of past and present mental illness was 24 %, higher for women with manic-depressive disorders, psychogenic (reactive) psychoses, and neuroses. The possible reasons for these and other findings are discussed.