Dysfunction of various hypothalamic systems may be the basis of a number of symptoms in Prader—Willi syndrome. The often abnormal position of the baby in the uterus at the onset of labour, the high percentage of infants with asphyxia and the high proportion of children born prematurely or post‐maturely may all be related to abnormal fetal hypothalamic systems, as the fetal hypothalamus plays a crucial role in labour. Abnormal luteinizing hormone‐releasing hormone neurones are thought to be responsible for the decreased levels of sex hormones, resulting in non‐descended testes, undersized sex organs and insufficient growth during puberty. A lack of growth hormone‐releasing hormone may also contribute to the short stature of patients with Prader—Willi syndrome. In addition, the aberrant control of body temperature and daytime hypersomnolence may result from hypothalamic disturbances. The number of oxytocin neurones – the putative satiety neurones – in the hypothalamic paraventricular nucleus is markedly decreased in Prader—Willi syndrome. This is presumed to be the basis of the insatiable hunger and obesity of patients with the syndrome. □ Growth hormone, growth hormone‐releasing hormone, hypothalamus, insatiable hunger, obesity, oxytocin, perinatal problems, Prader—Willi syndrome