Objective: In this study, to investigate the place of T cell-mediated immunity in the etiology of ADHD, for which we do not have enough information; we aimed to investigate the activity of DPP IV and ADA, which are T cell-related enzymes, and the relationship of these enzymes with ADHD symptoms in children with ADHD. Methods: Twenty-seven children aged 6 to 12 years with a diagnosis of attention deficit hyperactivity disorder and 27 children aged 6 to 12 years without any psychiatric disease were included in the study. Results: While serum ADA and DPP-IV activity were found to be statistically significantly higher in the group with ADHD. There was no statistically significant correlation between serum ADA and DPP-IV activities and CTRS-R-L and CPRS-R-L in both groups. Conclusion: We think that T cell mediated inflammation may play a role in the etiology of ADHD due to changes in ADA and DPP-IV levels in children.