Prader-Willi Syndrome (PWS) is the most frequent known genetic disorder of obesity. Hyperphagia is the main barrier to independent living in adults with PWS, and hitherto behavioural restraints and environmental modification are the only effective management measure. The emerging costs for professional care are immense. Thus, there is an urgent need for treatment which reduce appetite and food intake in this patient group. Agonists of the gut derived hormone GLP-1 which reduces food intake and causes weight loss due to slowed gastric emptying and through direct central effects. The aim of this pilot drug trial is to analyse the effect of a GLP-1 agonist on appetite regulating hormones, insulin secretion and energy expenditure before and after a meal.