The aim of our study is to compare uric acid plasma levels in patients with unipolar depression between those with Attention deficit hyperactivity disorder (ADHD) comorbidity and those without. Our hypothesis is that uric acid plasma levels may be higher in unipolar depressive patients with adult ADHD than without ADHD.Sixty four patients diagnosed with MDD were investigated, among which 28 patients had been diagnosed with ADHD according to DSM5. 28 patients were ADHD. 36 patients were diagnosed as not having ADHD. One of the criteria was including cases that had not started using medication for the current depressive episode. The control group (HC) consisted of 43 healthy staff members from our hospital who had no prior psychiatric admission or treatment history and matched with the patient group in terms of age and gender. Blood samples were obtained, and plasma uric acid levels were recorded in mg/dl after being rotated for 15min in a centrifuge with 3000 rotations and kept at -80°C.Uric acid plasma levels 5.1±1.6 in unipolar depression and ADHD group, 4.6±1.8 in unipolar depression group. Uric acid plasma levels were higher in the comorbid unipolar depression and ADHD group than in the unipolar depression and healthy control (HC) groups (F= 4.367, p= 0.037). There was no correlation between ADHD (predominantly inattentive type) and uric acid plasma levels (p>0.05).The limitation of this study is the small number of sample and one of the criteria was including cases that had not started using medication for the current depressive episode.The identification of a different etiologic process of biological markers may lead to a better understanding of the physiological mechanisms involved in drive and impulsivity and may suggest different potential targets for therapeutic intervention.