Prader-Willi syndrome is a chromosomal disorder characterized with obesity, mental retardation, short stature, small hands and feet and caused by deletion of chromosomal region of 15q.11-q13. Case: A 27 year-old-female patient presented with obesity, mental retardation, ulcers on the feet and hyperphagia. The patient was born by vaginal delivery (birth weight: 2800 grams), there were some feeding difficulties till 3 years old, after which obesity and hyperphagia developed. The parents were not consanguineous. Abnormal clinial findings: 128 kgs weight, 145 cms height (BMI:68.96). Pretibal minimal oedema (+) and bitemporal narrowing were noted. Breast development was Taner stage 2-3. Small hands and feet were detected. Laboratory findings: WBC and biochemical parameters were all in normal ranges except hyperglycemia. Hb A1c: 8.0 (4.8-6.5 %). Endocrinological investigations revealed hypogonadotrophic hypogonadism. Karyotype was established as 46,XX,del(15)(q11.2q13). The result was verified by FISH analysis with PWS/AS specific probe (SNRNP, Aappligene-Oncor). Differantial diagnosis of patients presenting with obesity and mental retardation should include Prader-Willi syndrome; we suggest that conventional chromosomal analysis and FISH analysis should be performed in such patients.