Behavioral interventions have had limited success in managing the chronic hyperphagia and obesity that are of presumed organic etiology in Prader-Willi syndrome. Thus, frequent foraging for food and covert consumption continue to be health-threatening problems for many Prader-Willi individuals. This case study was designed to replicate methods for assessment and treatment of food theft. A token program based on differential-reinforcement-of-other-behavior and response-cost eliminated theft in three hospital settings. Prior to discharge, the program was expanded to include contingencies on exercise behavior and weight loss, and staff from the subject's group home residence were trained to implement a modified program in the natural environment. Reduced food theft and continued weight loss were maintained in the group home and an apartment-living arrangement. A total of 81 lb (37 kg) was lost during a 2-yr period.