The inhibitory account of attention deficit hyperactivity disorder (ADHD) was tested by examining the performance of college-aged adults on a variety of inhibitory tasks. The poorer performance of adults with ADHD compared with controls on negative priming, stopping, and continuous performance tasks, combined with similar group performances on a test of working memory capacity, indicates a specific inhibitory deficit as opposed to a general limitation in attentional capacity. Overall results provide evidence for extending the inhibitory deficit hypothesis to adult ADHD, not only for mechanisms of response (or motor) inhibition but also for mechanisms of cognitive inhibition.