Prader-Willi syndrome is characterized by feeding difficulties in early infancy, followed in later infancy by excessive eating and gradual development of morbid obesity, unless externally controlled. All patients have some degree of cognitive impairment. Hypogonadism is present in both males and females. Short stature is common.PWS is associated with an abnormality of chromosome 15, it is generally considered not to be an inherited condition, but rather a spontaneous genetic birth defect that occurs at or near the time of conception.

People with PWS have a flaw in the part of their brain (the hypothalamus) that determines hunger and satiety (fullness). These people never feel full enough, so they have a continuous urge to eat. To compound this problem, people with PWS need considerably fewer calories than normal to maintain an appropriate weight. The obesity that results is the major cause of illness and death in this disorder. As in the general population, obesity in PWS can cause high blood pressure, respiratory difficulties, diabetes and other problems.