Prader-Willi Syndrome (PWS) is a complex neurogenetic disorder with considerable clinical variability, and is considered to be mainly the result of a hypothalamic defect. PWS is characterized by hyperphagia, obesity, mental retardation and hypogonadism from a young age. Hyperphagia is one of the most serious problems, which is organic in origin, inducing morbid obesity and leading to respiratory failure. Most studies attempting to control obesity in children with PWS by dietary management reported limited success due to difficulty in controlling foraging and food stealing. Here we report 16- and 20-year-old female patients with PWS who showed marked weight loss and improvement of respiratory failure by behavior modification and improvement of the environment.