The position taken in the 3rd edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM III), which holds that schizo-affective psychoses cannot be defined unequivocally, that no unambiguous criteria for these psychoses can be determined, and that they cannot be placed into either of the two main groups of functional psychoses, illustrates the present status of schizo-affective psychoses. The study of their position in relation to schizophrenia and manic-depressive psychoses is also essential: after all, the placing of a patient into either category plays a crucial role in the choice of treatment. For example, recent research on the success of lithium treatment in schizo-affective psychoses shows the practical relevance of this line of study. It seems that, like many other mental disorders, the internal structure of schizo-affective psychoses is highly heterogenous, and it is therefore not always possible to place them as a whole into either of Krae-pelin’s diagnostic categorie