Objective: Autistic disorder (AD) and Williams syndrome (WS) are neurodevelopmental disorders characterized by contrasting abnormal social behavior (the former, socially avoidant; the latter, outwardly social); nonetheless, there are individuals with WS who display some behaviors that are characteristic of AD. We quantified the extent to which autism spectrum disorder (ASD) behaviors were present in children with WS. Method: Twenty children with WS (27-58 months) and 26 age- and IQ-equivalent children with AD were administered the Autism Diagnostic Observation Schedule (ADOS). ADOS behaviors were compared between groups. Results: Two children with WS met DSM-IV criteria for AD, one of whom was also classified as having AD by the ADOS algorithm. Discriminant analysis of ADOS behaviors indicated that gesture, showing, and quality of social overtures best discriminated the groups. Conclusions: Although some children with WS demonstrated some ASD behaviors, and a minority of children with WS had coexisting AD, the symptom profile in WS was different from AD. Despite some deficits in communication behaviors, showing, and initiating joint attention, children with WS made social overtures and efforts to engage others, whereas children with AD tended not to do so. (Contains 1 table and 1 figure.)