Background and Methods: Unusual water intake and drinking behaviour has occasionally been observed in individuals with Prader-Willi syndrome (PWS). The aim of this study is to explore whether this observation is a part of the PWS phenotype and what the consequences may be. The parents of 51 individuals with PWS (age range 2-40 years) were asked by questionnaire to answer on past and present water intake, drinking behaviour, fluid preferences and medical treatment in their PWS-affected and unaffected children. Questionnaires with information on 47 PWS individuals and 17 without PWS were returned for analysis. The questionnaire information was complemented with information from the individual's medical records. Siblings to PWS individuals made up the control group. The study was approved by the regional medical research ethics committee. Results: During infancy, 36 (76%) individuals with PWS disliked water without any flavouring and had an extremely small daily intake of water. Seven individuals (15%) increased the daily water intake to unusually high amounts. In 45 the clinical PWS diagnosis was confirmed by molecular (genetic) testing: nine of them with a confirmed PWS diagnosis had a deletion of chromosome 15q11-13, in nine individuals no deletion was identified. The majority of individuals who increased their water consumption to extreme values belonged to the non-deletion group. Two in the non-deletion group developed hyponatraemia while receiving psychiatric medication. Conclusions: Infants with PWS seem to be predisposed to unusual drinking behaviour. They dislike and have an unusually small intake of pure water without flavouring, and most of them continue this even after infancy. Some individuals, especially those without deletion, increase their fluid intake and also accept pure water. They have an increased risk of developing water retention and severe hyponatraemia if exposed to medication known to cause side effects like the syndrome of inappropriate antidiuretic hormone secretion. Perhaps this behaviour is just secondary to overeating; perhaps it is a result of a dysfunction of the hypothalamic nuclei engaged in antidiuretic hormone production. (Contains 2 tables and 1 figure.)