Over-eating and resultant obesity is well recognized as a central feature of the Prader-Willi Syndrome (PWS). The eating behaviour of 13 subjects with PWS was been studied retrospectively over a 28-day period and also by direct observation when given free access to food. Changing cognitions normally associated with food intake (e.g. changes in hunger) were assessed using visual analogue scales (VAS) and by asking subjects to rate photographs of particular foods. Eight out of 13 subjects (61%) with PWS had to have their access to food severely restricted. Ten (77%) ate excessive amounts when given free access to food, and although feelings of 'hunger', 'desire to eat' and 'fullness' changed in the expected direction, these changes were delayed, compared to a control group, and only occurred after eating a significantly greater amount of food. Ratings of 'hunger' and 'fullness' started to return to pre-meal levels sooner than in the controls. The present authors consider that PWS is an example of genetic obesity secondary to an impaired satiety response. These observations have important implications for treatment.

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