Introduction: Nutritional obesity is the most common cause of obesity (95%) in childhood and adolescence. Morbid obesity (5%), which usually begins at an early age, can be caused by monogenic disorders, various genetic syndromes, endocrine diseases-disorders, central nervous system lesions or iatrogenic causes. Presentation of the patient: A female adolescent aged 13 is presented who was due to obesity referred to the Center for the Prevention and Treatment of Obesity in children and adolescents at the Special Hospital “Cigota” at Mt. Zlatibor. Low height/stature and dysmorphic features: a face with a narrow bifrontal diameter, almond-shaped eyes, strabismus, small hands and feet, delay in puberty development and lagging in psychomotor development have indicated the morbid obesity. Anamnestic data on hypotonia and difficulty in feeding, psychomotor lagging behind, obesity since the third year of age, triggered a suspicion of Prader Willy’s syndrome. The suspicion of this syndrome was confirmed by a molecular DNA analysis which indicated a deletion on the long arm 15q11.2. Conclusion: Obesity, endocrinolopathies, retardation in psychomotor development and behavioral disorders in people with Prader Willy’s syndrome require a complex multidisciplinary treatment. Early diagnosis and adequate treatment prevent the occurrence of complications and improve the quality and length of life of the patients.