Objective: The development of the concept of cycloid psychoses is the result of Kraepelins diagnostic dichotomy of the endogenous psychoses into dementia praecox and manic-depressive illness. This resulted in a number of unclassified psychoses that cannot be placed within this concept. Kleist called these psychoses marginal psychoses (“Randpsychosen”) to make clear that they do not belong to schizophrenia as proposed by Bleuler, but have to be placed ‘at the border’ of the affective psychoses.

Design and Methods: Leonhard described the cycloid psychoses as a group of nosological independent illnesses with an episodic course, intrasyndromal bipolarity and without residual symptoms in the long run. He described three clinical subtypes: anxiety-happiness-psychosis, confusion-psychosis and motility-

psychosis, each with a specific symptomatology. A large body of research now supports the validity of the concept of cycloid psychoses with respect to genetics, environmental causes and therapeutic strategies.

Results: Systematic scoring of the relevant psychotic symptoms showed a prevalence of cycloid psychoses of at least 1015% in psychotic patient groups. Antipsychotics play a minor role in the acute treatment and relapse prevention and the genetic endowment is quite different from bipolar illness and schizophrenia.

Conclusion: These and other data emphasize that cycloid psychoses have to be studied as a separate disease entity.