Children with attention-deficit hyperactivity disorder (ADHD) are impulsive, and that impulsiveness can be measured using a countermanding task. Although the overt behaviors of ADHD attenuate with age, it is not clear how well impulsiveness is controlled in adults with ADHD. We tested ADHD adults with an oculomotor countermanding task. The task included two conditions: on 75% of the trials, participants viewed a central fixation marker and then looked to an eccentric target that appeared simultaneous with the disappearance of the fixation marker; on 25% of the trials, a signal was presented at variable delays after target appearance. The signal instructed subjects to stop, or countermand, an eye movement to the target. A correct movement in this case would be to hold gaze at the central fixation location. We expected ADHD participants to be impulsive in their countermanding performance. Additionally, we expected that a visual stop signal at the central fixation location would assist oculomotor countermanding because the signal is presented in the "stop" location, at fixation. To test whether a central stop signal positively biased countermanding, we used a three types of stop signal to instruct the stop: a central visual marker, a peripheral visual signal, and a non-localized sound. All subjects performed best with the central visual stop signal. Subjects with ADHD were less able to countermand eye movements and were influenced more negatively by the non-central signals. Oculomotor countermanding may be useful for quantifying impulsive dysfunction in adults with ADHD especially if a non-central stop signal is applied.