Prader-Willi syndrome (PWS) is associated with an insatiable appetite and (often) other maladaptive behaviours (self-injury, sleep disorders, insistence on routines, and temper tantrums). Psychoses are not a recognised feature. Most affected people have a chromosome 15 abnormality (deletion, disomy, structural rearrangement, etc.). Three people with PWS who developed psychotic disorders in early adult life are described. The nature of the psychoses and the significance of the association are discussed.