The concept of cycloid psychosis has gained increasing acceptance during recent decades. Using the diagnostic criteria of Perris and Brockington, an intelligible delineation of a group of patients has been obtained. Few epidemiological data on cycloid psychosis have been reported so far. The objective of the present study was to describe the one-year incidence of cycloid psychosis in a clinical sample. The diagnostic registers of all patients hospitalized for a functional or an organic psychosis and discharged in the year 1983, in Lund, Sweden were investigated. 514 patients were identified of whom 83 were admitted to hospital for the first time. 29 of these patients had a functional psychosis and were below the age of 50. In this age group 7 cases (4 women, 3 men) fulfilled the diagnostic criteria of cycloid psychosis and thus constituted almost one fourth of all first admissions of functional psychoses that year. The one-year incidence for first admission in cycloid psychosis was 5.0 per 100,000 inhabitants in women and 3.6 per 100,000 inhabitants in men within the age group 15-50 years in the catchment area of 163,175 persons. We conclude that cycloid psychosis constitutes a considerable proportion of functional psychoses in both sexes.