In the present paper the author will try to elucidate the stability of nosological categories as revealed by a personal follow-up investigation of 301 previously hospitalized patients with paranoid psychoses. Since the terminology used will be the Scandinavian one, a short review of the diagnostic concepts will be given. The Scandinavian concept of schizophrenia is rather narrower than that employed in the rest of Europe, and especially narrower than that prevailing in the Anglo-American literature. Many American authors include in their concept of schizophrenia what may be called “the schizophrenic reaction types”. In the present paper, the concept of schizophrenia used by Langfeldt (1937, 1939), sometimes, referred to as “process schizophrenia” or “nuclear schizophrenia” is used. “Primary symptoms” occur in clear consciousness; these include disturbances of thinking, disintegration of emotions, autism, marked feeling of influence and passivity, massive depersonalization and derealization. As a rule the illness has an insidious development and a chronic course ending up with a greater or lesser degree of mental deterioration.