The Prader-Willi syndrome (PWS) is a genetically determined developmental disorder caused by abnormalities of the proximal region of chromosome 15q11-13. In a previous study, we reported that psychotic episodes, occurring in 16% of persons with PWS, had an onset in adolescence, never occurred in persons with paternal deletion, and were exclusively associated with maternal uniparental disomy (UPD) or imprinting abnormalities (IM). In order to gain a better understanding of the psychopathology and to further refine the psychiatric diagnosis, we describe in more detail the psychopathological manifestations of six adults with a history of psychotic episodes. All these individuals had a detailed psychiatric examination, including the use of the operational criteria (OPCRIT) checklist. An identifiable subtype of psychotic disorder was associated with PWS. Characteristics include early age of onset, acute onset, polymorphous, and shifting symptomatology and a need for psychiatric hospitalization. The presence of precipitating stress factors and a prodromal phase with physiological symptoms was reported in all patients. Current diagnostic categories do not allow an unequivocal psychiatric diagnosis.