Basing on the peculiarities of psychopathological presentations and dynamics of disease in 253 patients with an ICD-10 diagnosis of schizoaffective disorder (F25), the authors argue for the diagnostic validity of this disease. Schizoaffective psychosis (SAP) is considered in the context of endoform schizophrenic reaction in people with schizotypal personality with signs of reactive lability. In the dynamics of a schizoaffective episode, this reaction is formed in the period, but not in the structure, of endogenous phasic affective disorders and emerges itself as psychotic symptoms characteristic of schizophrenia which are independent from circulatory affective symptoms pathogenetically and pathokinetically. The clinical peculiarity and specifics of clinical-psychopathological presentations of SAP as a diagnostically valid entity are discussed. Diagnostic criteria of its differentiation from affective disorders and schizoaffective variants of episodic and recurrent schizophrenia are presented.