Context: A lack of inhibitory control has been suggested to be the core deficit in attention-deficit/hyperactivity disorder (ADHD), especially in adults. This means that a primary deficit in inhibition mediates a cascade of secondary deficits in other executive functions, such as attention. Impaired stopping has been claimed to support the inhibition hypothesis. However, executive functions such as inhibition and attention are hard to disentangle. Objective: To use event-related potentials in adult patients with ADHD to show that impaired stopping is associated with abnormalities of attention. Design: The stop signal task was presented to 24 adults with ADHD combined subtype and 24 controls. Stop event-related potentials are distorted by overlap from event-related potentials to other stimuli in close temporal proximity, but we applied a method (Adjar level 2) to effectively remove this overlap. Results: In line with an inhibitory control deficit, the stop signal reaction time was longer in adults with ADHD (F1,46=7.12, P<.01) whereas there was no significant difference for go stimulus reaction time. Overlap-free stop event-related potentials revealed smaller stop P3s in adults with ADHD (F 1,44=4.20, P<.05). In children with ADHD, this has been interpreted to reflect deficient inhibitory control. However, controls were also found to have larger early responses in the auditory cortex (N1) when stop signals resulted in successful stops, relative to failed stops, signifying increased attention (F1,23=11.88, P<.01). This difference was completely absent in adults with ADHD. Conclusions: Disturbed attentional processing of the stop signal contributed to impaired stopping in adults with ADHD. This finding may have implications for treatment. ©2005 American Medical Association. All rights reserved.