The psychiatric symptomatology of people with Prader-Willi syndrome (PWS) has mainly been described in case reports and some large-scale descriptive studies. Unfortunately, there is still no systematic description of all the psychiatric symptoms which accompany this chromosomal error. Symptoms of mood disorder and anxiety dominate the picture of PWS, although some reports also mention psychotic symptoms with variations in prevalence of between 15% and 60%. The present paper presents six case reports of adult male subjects with a diagnosis of PWS and psychiatric symptoms who fulfill the criteria for cycloid psychosis (ICD-10 F.23.0). This psychotic disorder requires a specific psychopharmacological approach with mood stabilizing agents, particularly Lithium. It is concluded that subjects with PWS may be especially vulnerable to the development of cycloid psychosis, which suggests the existence of a specific 'psychopathological phenotype'.