Based on sophisticated clinical descriptions and characteristic symptom patters occurring during the long-term course of psychiatric diseases, Leonhards classification requires that all the clinical features fit for a diagnosis to be made. Always using his own life-long case studies, Leonhard divided the endogenous psychoses into five main nosological groups: on the one hand, the prognostically favourable unipolar affective psychoses, bipolar affective psychoses and cycloid psychoses, and, on the other hand, the prognostically unfavourable unsystematic and systematic schizophrenia. In the meantime, the reliability and validity of this classification have been confirmed by scientific studies and early results provide guidelines for the different origins and varied therapeutic approaches for the specific disease profiles.