Background: The present study aimed to compare blood neutrophil, lymphocyte, platelet, neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), erythrocyte and systemic inflammatory index levels in children with attention deficit hyperactivity disorder (ADHD) with typically developing (TD) healthy controls and to investigate the relationship between these levels and executive functions. Materials and Methods: In this study, 79 ADHD children aged 8-14 years who did not use medication and 34 healthy controls were included. Participants’ blood levels were based on routine complete blood count analysis. Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) was used for clinical diagnosis in children, and the Stroop color test and serial digit learning test were applied to evaluate executive functions. Results: No significant difference was found between the groups in neutrophil, lymphocyte, platelet, NLR, PLR, erythrocyte, and systemic inflammatory index levels. Compared to TDs, patients with ADHD were observed to perform significantly worse on executive function tests. Platelets, neutrophils, and lymphocytes were positively correlated with Stroop first section correction scores, platelets were positively correlated with Stroop second section time scores, lymphocytes were positively correlated with Stroop second section correction scores, NLR was negatively correlated with Stroop second section correction scores and platelets was positively correlated with Stroop third section time scores. Platelets and lymphocytes were positively correlated with Stroop section fourth time scores and NLR was positively correlated with Stroop section fourth error scores. Conclusion: This study suggests that serum lymphocyte, neutrophil, NLR, and platelet levels may be associated with impaired executive tests in ADHD.