Objective: There are few studies evaluating the biochemical basis of Adult Attention Deficit /Hyperactivity Disorder (A-ADHD). In the present study, we evaluated whether nitric oxide (NO), an oxidant, level and superoxide dismutase (SOD), an antioxidant, activity are associated with A-ADHD or not. This study also aims to evaluate the NO levels and SOD activities, which were already measured and found to be associated in other psychiatric disorders, in A-ADHD and hopes to find some clues underlining the biological basis of the disease. Method: Twenty A-ADHD patients from Gaziantep University Sahinbey Research Hospital, Psychiatry Clinic, diagnosed according to The Turkish version of Adult ADD/ADHD DSM IV- Based Diagnostic Screening and Rating Scale by two psychiatrists (H.A.S. and S.S.), and twenty one healthy volunteer controls were included. Blood samples were collected, NO levels and SOD activities were measured. Results: The mean NO levels in patients (181.39 ± 35.85 μmol/L) were significantly higher than those of controls (40.14± 7.71 μmol/L) and SOD activity of patients (7.00± 1.34 U/L) was significantly lower than controls (11.18 ± 1.31 U/L) (t= 17.64, df= 39, p< 0.01 and t= -10.09, df= 39, p< 0.01 respectively). Conclusions: Remarkable high levels of oxidant NO, and low SOD activities suggest an oxidative imbalance in A-ADHD. This is the first study evaluating the oxidative metabolism in A-ADHD. Our findings may pioneer the further clinical enzymology and biochemical studies on that disorder.