Objective: The purpose of this study was therefore to evaluate serum heme oxygenase-1 (HO-1), kelch-like ECH-associated protein 1 (KEAP1), and nuclear factor erythroid 2 related factor 2 (NRF2) levels in children with attention deficit hyperactivity disorder (ADHD) and to reveal their association with the severity of ADHD. Methods: The patient group consisted of 41 children aged 612 years, first diagnosed with ADHD and the control group consisted of 34 age- and gender- matched children. ADHD was diagnosed by child psychiatrists during clinical interviews based on the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5). The parents of the children in the ADHD group were also given the Turgay DSM-IV-based Disruptive Behavior Disorders Child and Adolescent Rating and Screening Scale (T-DSM-IV-S). Serum HO-1, KEAP1, and NRF2 levels were determined in the biochemistry laboratory using the ELISA technique. Results: HO-1 levels in the patient group were significantly lower than in the control group, while KEAP1 levels were significantly higher, but no significant difference was observed between the groups in terms of NRF2 (p=0.041, p [Anadolu Psikiyatri Derg 2020; 21(3.000): 319-326]