This study examines the nature, severity and correlates of non-food obsessions and compulsions in 91 people with Prader-Willi syndrome (PWS) aged 5-47 years (mean age = 19 years). Prominent symptoms, seen in 37-58% of the sample, included hoarding; ordering and arranging; concerns with symmetry and exactness; rewriting; and needs to tell, know or ask. A remarkably high proportion of participants had moderate to severe symptom severity ratings; 64% showed symptom-related distress, and 80% showed symptom-related adaptive impairment. The study also compared obsessive-compulsive symptoms in 43 adults with PWS to age- and sex-matched non-retarded adults with obsessive-compulsive disorder (OCD). The PWS and OCD groups showed similar levels of symptom severity and numbers of compulsions; they also showed more areas of symptom similarity than difference. Increased risks of OCD in persons with PWS are strongly indicated. Implications are discussed for pharmacotherapy, behavioral therapy and family support.