Objective: To evaluate the biochemical basis of adult attention-deficit hyperactivity disorder (A-ADHD), we compared lipid peroxidation status in the plasma of A-ADHD patients, and that of control subjects without A-ADHD by quantifying the levels of malondialdehyde (MDA), an end product of fatty acid oxidation. We aimed to examine the association between MDA and A-ADHD. Method: The study comprised 20 A-ADHD patients from Gaziantep University Sahinbey Research Hospital Psychiatry Clinic, diagnosed by 2 psychiatrists (H.A.S. and S.S.) according to the Turkish version of the adult ADD/ADHD DSM-IV-Based Diagnostic Screening and Rating Scale, and 21 healthy volunteers. Malondialdehyde levels were measured in plasma samples of both study groups, Results: The mean (standard deviation [SD]) MDA levels in patients (2.44 [0.84] nmol/mL) were significantly higher than those of control subjects (0.36 [0.20] nmol/mL) (t = 11.013, of = 39, p < 0.01). MDA levels were correlated with overall number of criteria met (n = 20, p = 0.01, Ro = 0.56) and total hyperactivity/impulsivity score(n = 20, p = 0.02, Ro = 0.51). Conclusion: The fact that MDA levels were increased in A-ADHD could be an indication of increased oxidative stress in this disease. We suggest that such changes may have a pathological role in A-ADHD. This is the first study evaluating the MDA levels in A-ADHD, and our findings may provide a scientific guide for the further clinical enzymologic and biochemical studies on this disorder.