This paper describes an intervention with a man who has Prader-Willi Syndrome (PWS) and mild learning disabilities in a residential setting. PWS is a chromosomal disorder that is characterized by a wide range of behavioural characteristics including overeating and challenging behaviour. The intervention was based on a range of principles derived from a motivational interviewing approach, which were embedded in a broader behavioural framework that was designed to assist in weight reduction and reduce challenging behaviour. Records of weight and challenging behaviour kept over time suggest that the approach had relatively little impact on overall weight. However, levels of challenging behaviour were reduced quickly and have been maintained at a stable and lower level. Weight has also been maintained at a reasonably constant level without confrontation with staff or excessive restrictions on the individual concerned. Relationships between the individual and staff in the home also improved over the course of the intervention.