Formal thought disorders are usually regarded as a central symptom of schizophrenic psychoses, but nevertheless as diagnostically non-specific. Karl Leonhard, therefore, differentiated in his classification of endogenous psychoses between a whole lot of diagnostically specific types of formal thought disorders. He described two psychoses, of which formal thought disorder is the main psychopathological feature, i.e. confusion psychosis and cataphasia. By means of an analysis of tape-recorded utterances of patients with such psychoses we demonstrate the phenomenological differentiation of the distinct types of thought disorder in these psychoses. In confusion psychosis the disturbance concerns the thematic organisation and target-oriented direction of the train of thought. Characteristically, an "incoherence of thematic choice" results, reflecting a disturbance on the level of organisation of discourse. The concepts, ideas and themes itself are organised in a logically correct manner and are comprehensible. The thought disorder in cataphasia, however, shows severe logical faults in the connection of concepts and ideas, as a result of which single themes and ideas become a logical and incomprehensible. Moreover, cataphasic thought disorder goes beyond the level of discourse organisation and extends to disturbances of linguistic functions including mistakes on a semantical and syntactic level. These observations correspond well with the widely accepted assumption of a modular organisation of cognitive and linguistic processes. The thought disorder in prognostically unfavourable cataphasia concerns "basal" logical, semantic and syntactic systems, which remain uninfluenced in the prognostically favourable confusion psychosis. In the latter, therefore, the disorder is restricted to the higher functions of organisation of discourse.