This paper examines nosological categories relating to borderlines between psychosis and other clinical categories, introduced by Polish psychiatrists in the interwar period. In the United States, the discussion about the borderline between neuroses and psychoses was urged by the 1938 article by psychoanalyst Adolph Stern. In Poland, nosological categories regarding the borderline between neuroses and psychoses were proposed by Adam Wizel, Maurycy Bornsztajn, Jan Nelken, and Władysław Matecki. Wizel coined the term ‘underdeveloped schizophrenia’, Bornsztajn introduced ‘schizothymia reactiva’ and ‘hypochondriac (somatopsychic) schizophrenia’, Nelken described ‘mild schizophrenia’, first introduced by Moscow psychiatric school of Rosenstein, and Matecki presented the category of neurosis-like (pseudo-neurotic) schizophrenia. Additionally, Julian Dretler, after studying the borderline between schizophrenia and manic-depressive psychosis, coined the term ‘mixed psychosis’ and expressed conviction that it is an independent nosological entity. Like in the United States, the majority of Polish pioneers of the nosological studies of borderline cases were influenced by psychoanalysis. As a consequence of World War II and the new regime, which forced dialectical materialism and Pavlovism as an official ideology of psychiatry and condemned psychoanalysis, the categories presented in the article became forgotten and have not impacted Polish psychiatric nosology.