Prader-Willi syndrome (PWS), a genetic form of mental retardation, involves a myriad of physical and behavioral problems. Poor social adjustment has been reported, but the origin of this difficulty is unknown. The Social Attribution Task, a measure of one's ability to make appropriate social attributions from an ambiguous visual display [Klin (2000) Journal of Child Psychology and Psychiatry, 33(5) [861-876] was administered to study participants with PWS, participants with pervasive developmental disorder and an IQ matched comparison group with no known syndrome. The participants with PWS performed significantly more poorly than participants with comparable intellectual ability, and not significantly differently from the group of participants with a pervasive developmental disorder. Poor performance on this task by the PWS participants suggests an underlying difficulty interpreting social information that is presented visually, which may be a critical factor in the impairment in social functioning in this population.