This study investigated the movement preparation (reaction time) and movement execution (movement time) of children with and without ADHD by manipulating the uncertainty of occurrence. Participants performed a seated lower extremity choice response time protocol, which contained either 10% catch trials or 30% catch trials along with 27 empirical stimuli to one of three target directions. Results indicated that children with ADHD were significantly slower at processing lower extremity movements than their peers for the condition with increased number of catch trials, but not the condition with fewer catch trials. These findings suggest that children with ADHD are more affected by the uncertainty of an empirical stimulus during the preparation phase of a movement response than their age-matched peers are. (Contains 2 figures.)