Williams syndrome (WS) is a genetic disorder that causes general cognitive and developmental delays. Compared to persons with Down syndrome (DS) at the same developmental level, individuals with WS generally exhibit superior expressive language abilities, but have difficultly with tasks that require the visual control of movement. Recently it has been suggested that this latter problem reflects a deficit in dorsal stream function. In the present study, this hypothesis was investigated by examining the kinematics of rapid aiming movements. The performance of the participants with WS (
n= 4) was compared to the performance of participants with DS ( n= 8), with undifferentiated developmental delays ( n= 8), and from the general population ( n= 8). In partial support of the dorsal deficit hypothesis, the results suggest that, compared to people from the other groups, the participants with WS had difficulty in preparing their movements on the basis of the visual and other information available to them. This was particularly evident in their inability to properly scale movement velocities to the amplitude of the movements and in the number of discrete corrections made during movement execution.