Karl Leonhard's Classification of Endogenous Psychoses is based on sophisticated clinical descriptions and characteristical symptom patters occurring during the long-term course of psychiatric diseases. A diagnosis can be made only if all the clinical features fit, i.e. a special diagnosis may not be made if one characteristic symptom is lacking. This is the main and important difference between Leonhard's classification and the operational diagnostic systems that require the presence of some but not all specific symptoms of a symptom cluster. Leonhard took over the essential features since the early days of Psychiatry and found, always based on own life-long case studies, that the endogenous psychoses have to be divided into five main nosological different groups: on the one side the prognostically favourable unipolar affective psychoses, bipolar affective psychoses and cycloid psychoses and on the other side the prognostically unfavourable unsystematic and systematic schizophrenias.
Meanwhile its reliability and validity have been confirmed by scientific studies and early results provide guidelines regarding the different origins and varied therapeutic approaches for the specific disease profiles.