Prader-Willi syndrome is a disorder caused by loss of genetic material from proximal arm of chromosome 15. An important characteristic of Prader-Willi syndrome is a continuous sense of hunger that usually begins at around 2 years of age. Persons with Prader-Willi syndrome want to eat constantly because they never feel full (hyperphagia) and usually have trouble controlling their weight. Many problems of Prader-Willi syndrome are due to obesity. The common features of this disease are mental retardation, short stature, small hands and small feet etc. Prader-Willi syndrome has helped to define not only the phenotypic features of the syndrome associated with the various genotypes but also to anticipate clinical and psychological problems that occur at each stage during the life span.