This fact sheet uses a question-and-answer format to summarize what is known about Prader-Willi Syndrome (PWS), a complex genetic disorder resulting in short stature, mental retardation or learning disabilities, incomplete sexual development, characteristic behavior problems, low muscle tone, and an involuntary urge to eat constantly, which results in obesity. Questions and answers address the following topics: what Prader-Willi Syndrome is; the inheritability of PWS; incidence of PWS (about 1 in 14,000); infants with PWS and development of the typical eating disorders; the defect in the hypothalamus which causes the overeating; ways to control the PWS individual's eating; other characteristics of PWS; the genetics of PWS; typical behavior problems of people with PWS; types of intervention and care which allow a reasonably normal life; the importance of early diagnosis; how to determine if one's child has PWS; and sources of more information. (DB)