Objective: The prevalence of Adult attention-deficit/hyperactivity disorder (A-ADHD) has been estimated as to be around 5%. There are several hypotheses regarding the etiology of ADHD. Although numerous researches have been conducted regarding neurobiology of pediatric ADHD, A-ADHD studies were relatively few. Many studies have indicated that oxidant mediated neuronal damage may play a role in the pathophysiology of various psychiatric disorders. Recently, oxidative stress has been studied in A-ADHD and there is an accumulating evidence to support its' role. In the present study, we aimed to evaluate catalase (CAT) and thiol levels in A-ADHD patients and compare with healthy controls.