This study aimed to examine potential disparities in hematologic inflammation parameters between children diagnosed with attention deficit hyperactivity disorder (ADHD) and their healthy counterparts and to determine whether atomoxetine treatment induced any alterations in inflammation indicators. This case–control study involved 43 children aged 6–13 years, 22 diagnosed with ADHD for the first time, and 21 healthy children. In all children, complete blood count and albumin, C-reactive protein (CRP), thyroid stimulating hormone (TSH), free thyroxine (free T4), folate, vitamin B12, aspartate aminotransferase (AST), alanine transaminase (ALT), creatinine and urea values were performed. Children with ADHD were started on atomoxetine treatment, and one month later, the blood test was repeated for those who commenced treatment. Neutrophil (p = 0.005), platelet (PLT) (p = 0.002), neutrophil/lymphocyte ratio (NLR) (p = 0.001), platelet/lymphocyte ratio (PLR) (p < 0.001), systemic immune /inflammation index (SII) (p < 0.001) and pan-immune-inflammation value (PIV) (p = 0.025) parameters were found to be significantly higher than the control group, while the lymphocyte value (p = 0.001) was found to be significantly lower. In those in the ADHD group, lymphocyte (p = 0.041) and albumin (p = 0.027) values increased significantly after treatment. The results of this study show the increase in inflammation in drug-naive ADHD patients and the partial improvement after treatment. However, there is a need to evaluate inflammation in larger samples after longer-term treatments and follow-ups.