Alzheimer's disease (AD) is well-known as a condition of old age, prevalence rising with age from about 70 years. However, some groups appear to be at risk from a much earlier age, for example people with Down's syndrome. Recently, in pathological studies of people with Prader-Willi syndrome (PWS) who died aged over 40 years, signs of AD have been found in three brains, the severity of pathology increasing with age. We propose to interview parents and carers of people with PWS aged over 40 years to ask about any signs of developing AD. This involves asking about changes in behaviour, personality, cognition, and competence in self-care. Our aim is to assess the risk of early-onset AD in people with PWS. This is relevant because life expectancy in PWS is rising as a result of early diagnosis and interventions to prevent obesity and its health threatening complications. Early diagnosis of AD has the potential to prolong the non-dependent life of people with the condition by use of suitable medication.