The purpose of the study was to characterize repetitive phenomena in Williams syndrome (WS). The parents of 60 subjects with WS completed the Yale-Brown Obsessive Compulsive Scale (Y-BOCS) or Children's Y-BOCS, the Yale Global Tic Severity Scale, the Stereotyped Behavior Scale, and the Spence Children's Anxiety Scale--Parent Version. Nineteen males and 41 females participated in the study. Six subjects (10%) had obsessions only, six (10%) had compulsions only, and eleven (18%) had at least one obsession and at least one compulsion. None of the subjects had tics. Fifty subjects (83.3%) endorsed at least one stereotypy. Increased anxiety was associated with increased severity of obsessions, but not severity of compulsions or stereotypies.