A 3-year prospective follow-up investigation and a family study were carried out in patients fulfilling the diagnostic criteria for cycloid psychotic disorder proposed by Perris and Brockington in 1981. The results show that cycloid psychosis differs in several respects from schizoaffective disorders as defined by current diagnostic criteria used in the United States. Furthermore, they seem to suggest that most cases of cycloid psychosis are not variants of either schizophrenia or major affective disorders.