Objective: We hypothesized that patients with ADHD were typified by distress more than by functional difficulties. Thus, a decline in state anxiety while performing a cognitive task when taking methylphenidate would discriminate between ADHD patients and controls. Method: State anxiety and cognitive performance on a continuous performance test were assessed in ADHD patients and controls with and without taking methylphenidate. Results: State anxiety and cognitive performance improved from baseline in 36 ADHD adults after taking methylphenidate. In 25 controls, cognitive performance improved, but state anxiety did not abate after a recess. In two additional studies, 5 controls were evaluated at baseline and after receiving methylphenidate, and showed improvement in cognitive assessment but not in state anxiety. Five ADHD adults were assessed at baseline and after a recess, and showed no improvement. Conclusion: Our results support the hypothesis that adult ADHD patients are characterized by distress and the relief of this distress under effective therapy as expressed by a decline in state anxiety while they perform a cognitive task.