The objective of the present study on cycloid psychosis was to describe the incidence and risk in a defined population sample. We therefore re-evaluated specific diagnostic groups in the 1947 cohort of the Lundby Study. Three female cases were identified as cycloid psychosis according to the diagnostic criteria of Leonhard, Perris and Brockington. No men were found. The incidence rate (per 100 observation years) was found to be 0.016% for women. The cumulative probability i.e. risk, up to 60 years of age was calculated to be 0.7%. Incidence rate and risk for cycloid psychosis in women was thus about half of the corresponding values for schizophrenia as described for the same population in a parallel study. We conclude that cases of cycloid psychosis constitute a substantial proportion of female psychotic patients.