The author analyzes the features of the syndrome of «delusional fantasies» (delusion-like fantasies) in the concept of K. Birnbaum. According to this concept, psychopathic constitutions, due to the presence in the structure of dissociation, are the basis for the development of delusions per se. «Dreamer» and «vershroben» types are predisposed to the syndrome of «delusional fantasies». It is emphasized that initially the syndrome of «delusional fantasies» was described as a peculiar phasic episode of delusions in psychopaths, aged between 20—30 years. In the current domestic literature, delusion-like fantasies are regarded as non-delusional bizarre fantastic ideas in children and adolescent patients.