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The study confirms the distinct behavioural phenotype of PWS. Specific behaviours occurred significantly more frequently in PWS, compared with an age and BMI matched learning disabled comparison group. A factor analysis of the behaviours involved resulted in three factors that we hypothesized to be independent, and to arise from different mechanisms.

The longitudinal course of psychiatric illness and response to medication in people with PWS is fully described. Further research is needed regarding the effect of psychotropic medications, particularly mood-stabilizing medication. These data will enable informed decisions to be made regarding management options and provide information on the possible long-term outcome of illness.

The medical findings from a population‐based study of Prader‐Willi syndrome (PWS) are discussed (in which birth incidence of PWS was estimated at 1:22 000 and death rate at over 3% per annum). In this study the prevalence of specific medical disorders that might account for a shortened life expectancy were investigated. Of all people with a possible diagnosis of PWS, only those meeting clinical criteria and/or with a confirmed genetic diagnosis were included in the study. Sixty‐six individuals, 40 males and 26 females with a mean age of 19 years (range of 0 to 46 years) agreed to participate in the population‐based study group. A prevalence rate of 25% for non‐insulin dependent diabetes mellitus (NIDDM) was found in adults. Mean age at onset was 20 years. Those with NIDDM had a higher past maximum body weight and a greater likelihood of positive family history. Nearly 50% across the age groups reported a history of recurrent respiratory infections. High rates of fractures (29%), leg ulceration (22% in adults), sleep disorders (20%), and severe scoliosis (15% in childhood) were also reported. It is postulated that hypotonia is a possible contributory factor to the risk of strabismus, scoliosis, and respiratory infections. Other causes of morbidity, in particular the high rates of NIDDM, may be due to a failure to manage over‐eating resulting in severe obesity. Early diagnosis and clear guidance to families about these risks and how they might be prevented is recommended. It is hypothesized that the high pain threshold may result in the presence of some illness not being apparent.