Objective: Attention deficit hyperactivity disorder (ADHD) is associated with poor inhibition of prepotent responses and deficits in distractor inhibition, but relatively few studies have addressed inhibitory control of proactive interference (PI) in individuals with ADHD. Thus, the goal of the present study was to evaluate resistance to spatial and verbal PI in adults with ADHD. Method: Adults with ADHD (n = 32) and adults without ADHD (n = 32) performed a spatial inhibition of return (IOR) task and a semantic IOR task. Results: Adults with and without ADHD showed effective spatial IOR. On the semantic IOR task, adults without ADHD showed semantic inhibition, whereas adults with ADHD showed semantic facilitation (positive priming) instead. Conclusion: Adults with ADHD may have domain-specific deficits in resistance to proactive interference. Research has theoretical and clinical implications for adults with ADHD. (Contains 2 tables.)