Animal experiments have shown that the parvocellular oxytocin (OXT) neurons of the hypothalamic paraventricular nucleus (PVN) inhibit food intake. In the present study, the PVN and its OXT neurons have been investigated in an extreme human eating disorder, i.e. the Prader-Willi syndrome (PWS). PWS patients are characterized by gross obesity, insatiable hunger, hypotonia, hypogonadism, and mental retardation. The PVN of 5 PWS patients (2 males and 3 females), varying in age between 22-64 yr, and 27 controls (14 males and 13 females) without any primary neurological or psychiatric diseases was morphometrically investigated after conventional staining with thionine and immunocytochemical staining for OXT and vasopressin (AVP). The thionine-stained volume of the PVN was 28% smaller in PWS patients (P = 0.028), and the total cell number was 38% lower (P = 0.009). The immunoreactivity for OXT and AVP was decreased in PWS patients, although the variability within the groups was high. A strong and highly significant decrease (42%; P = 0.016) was found in the number of OXT-expressing neurons of the PWS patients. The volume of the PVN-containing OXT-expressing neurons decreased by 54% (P = 0.028) in PWS. The number of AVP-expressing neurons in the PVN did not change significantly. The OXT neurons of the PVN seem to be good candidates for playing a physiological role in ingestive behavior as "satiety neurons" in the human hypothalamus.