Older individuals with Prader-Willi syndrome have rarely been documented. This report describes the physical characteristics, health problems, cognition, psychosocial adjustment and impact on the family of 232 adults with the syndrome, ranging in age from 16 to 64 years. The sample showed that Prader-Willi syndrome occurs with equal frequency in both sexes. Most were short, overweight, cognitively impaired, emotionally labile, had poor gross motor skills and were always hungry. Males were taller and heavier than females, and both sexes were far shorter and heavier than US norms. Micropenis and cryptorchidism in males and primary amenorrhea, late menarche and irregular menstrual cycles in females indicated hypogenitalism in both sexes. Of 106 with chromosome analysis, 54 had an abnormality on chromosome 15, primarily a deletion; the others had normal chromosomes.