Research suggests that people with a Williams syndrome (WS) or Down syndrome (DS) diagnosis display an increased prevalence of autism spectrum disorder (ASD) when compared to the general population. This study aimed to examine characteristics of ASD in a group of children with DS or WS. Results suggest that children with DS and WS exhibit higher levels of autism symptoms than the general population, particularly in the area of unusual behaviors, and that these elevations are not solely due to deficits in adaptive behavior. There are many possible explanations for these elevations, such as issues with measurement, etiological overlap, or similar behavioral phenotypes. More research is needed to further our understanding of the overlap of ASD symptoms in these populations.