The study examined behavioral reactions to sound, including startle eye blinks, in young children with Williams syndrome (WS) using video-based observational techniques. Participants were 21 children with WS and 20 children with other developmental disabilities of mixed etiology between the ages of 2.5 and 6. Groups were matched for chronological age and developmental level. All children participated in a semi-structured play interaction including exposure to mild intensity sounds as emitted from conventional toys. Overall, 90% of the children in the WS group were observed to exhibit overt behavioral reactivity to mild intensity sounds, compared to only 20% in the mixed etiology group. Examination of the temporal sequence indicated that children with WS generally exhibited these behaviors before exposure to sound stimuli, suggesting a relation to anticipatory anxiety. Children with WS also exhibited significantly greater acoustic startle eye blinks, often viewed as an indication of heightened emotional state. Taken together, the current findings confirm the presence of heightened reactivity to sound in WS, behaviors previously investigated using parent report alone. The observed behaviors and their potential relation to anxiety are also discussed.