Prader-Willi syndrome is a multi system disorder characterized by neonatal hypotonia, later obesity, hyperphagia and mental retardation. It occurs sporadically, either as a result of microdeletion of chromosome 15p (70%) or as a result of maternal disomy of chromosome 15 (30%). The major problems encountered by parents are those of hyperphagia, food-seeking and obesity, and conduct disorder, particularly tantrums or oppositional behaviour.