The concept of cycloid psychotic disorder.

The concept of cycloid psychosis or cycloid psychotic disorder has been used in the European psychiatric literature for almost half a century. However, it has now for the first time been comprised into the 10th revision of the World Health Organization's International Classification of Diseases (ICD-10) that is currently in the phase of field trials. In this article evidence is presented that supports the independence of cycloid psychotic disorder from other major psychotic disorders. In particular, evidence is presented in favour of the clinical and predictive validity of the cycloid psychosis construct. Issues related to treatment are discussed and suggestions for future research are given.