Background. Despite its clinical relevance, the diagnosis of cycloid psychosis has been relatively neglected in the psychiatric literature and in the current nosological systems. This study examined the clinical validity and nosological status of the cycloid psychosis concept.

Method. Six-hundred and sixty psychotic in-patients were assessed for psychosis-related variables and diagnosed according to DSM-III-R, DSM-IV, ICD-10 and the Perris & Brockington criteria for cycloid psychosis. The cycloid psychosis diagnosis (N=68, 10·3%) was examined in regard to its discriminant validity, concordance with other psychotic disorders, and predictive validity in relation to schizophrenia and psychotic mood disorders. To address putative heterogeneity within cycloid psychosis, affective (N=38) and non-affective (N=30) subgroups were examined.

Results. Cycloid psychosis had good discriminant validity regarding other psychoses (95% of correctly classified cases) and poor concordance with individual diagnoses from the formal diagnostic systems (κ<0·22). Cycloid patients had levels of psychotic, disorganization and first-rank symptoms comparable to schizophrenia, and levels of affective symptoms in-between schizophrenia and mood disorders. Regarding most clinical variables and morbidity risk of mood disorders, cycloid psychosis was closer to mood disorders. Cycloid psychosis had higher psychosocial stressors than schizophrenia and mood disorders. Affective and non-affective groups of cycloid psychosis differed in a number of variables indicating an overall better outcome for the non-affective group.

Conclusions. Cycloid psychosis does not correspond closely to any DSM-III-R, DSM-IV or ICD-10 category of psychosis, and more specifically this nosological concept is not well represented by the different formal definitions of remitting psychotic disorders. Cycloid psychosis seems to be an heterogeneous condition in that affective and non-affective subgroups can be differentiated.