Prader-Willi Syndrome (PWS) is a complex multisystem disorder with many manifestations interrelating to hypothalamic deficiency. PWS is characterized by neonatal hypotonia, hypogonadism, hyperphagia and onset of obesity in early childhood, developmental disability, mild-moderate mental retardation, characteristic facial appearance, short stature, and behavioural abnormalities (skin picking, tantrums, stubbornness). PWS results from the absence of the normally active paternally inherited genes on chromosome 15q11-q13. It is estimated to occur at a frequency of 1 in 10 000 to 20 000 individuals. Obesity and behavioural problems are primary reasons for morbidity and mortality. The patients with PWS are at increased risk for several mental health and behaviour problems. In addition to the syndrome’s characteristic food seeking behavior and hyperphagia, they also have increased risks of nonfood-related compulsive behaviours, increased rates of tantrums, oppositionality, aggression and psychotic disorder or affective illness with a psychotic component. In some sources, compulsivity is considered intrinsic to the Prader-Willi genotype, therefore, PWS seems to predispose many individuals to obsessive-compulsive behaviour, if not full-blown obsessive-compulsive disorder (OCD). A 10-year-old female admitted to our child psychiatric outpatient clinic with moderate mental retardation, behavioural problems, obesity, and skin picking. The presentation of the clinical picture was attributed to impulse control problems resulting from mental retardation. Behavioural and psychopharmacological therapies, including risperidone, aripiprazole, zuclopenthixol, haloperidol and methylphenidate were used with limited success. During the follow up for metabolic problems, she was referred to department of Medical Genetics, where PWS diagnosis was established. Although majority of the children are diagnosed in early childhood, this is the 10 years old-case displaying behavioral problems, having the diagnosis of PWS at a relatively later age. Especially, all hypotonic infants with clinical suspicion of PWS should go under molecular testing, so that an early diagnosis can be established. Early diagnosis is important since the delay may have negative consequences on the progression and the management of the symptoms of the disorder. When the patients with central obesity and compulsive behaviours accompanying mental retardation is presented to child psychiatry clinics, PWS should be considered as a differential diagnosis. The purpose of this presentation is to emphasize the importance of recognition of patients in terms of PWS.