Introduction: The aim of this study was to evaluate the effects of intraoperative and postoperative theophyl- line infusion on inflammation in children who underwent open heart surgery with cardiopulmonary bypass (CPB) for congenital heart diseases. Patients and Methods: This randomized controlled study was carried out with 140 children who underwent open heart surgery with CPB in a tertiary hospital. The patients were randomly enrolled into two groups (the- ophylline group= 70, control group= 70). Following the induction of anesthesia, theophylline infusion was applied to theophylline recipients from the beginning of the operation to the end of the 24th hour after surgery via a central venous catheter (CVC). The patients in the control group were infused with 0.9% NaCl solution at the same volume and duration via CVC. Results: Postoperative red blood cell distribution width was significantly higher than preoperative values in the theophylline group (p= 0.003), but no significant difference was found in the control group (p= 0.188). In both groups, postoperative vasoactive-inotropic scores, and lymphocyte and platelet counts were lower than preoperative measurements (p< 0.001 for each in both groups). While preoperative white blood cell counts were similar, postoperative values were found to be lower in the theophylline group (p= 0.022). Conclusion: In pediatric patients undergoing open heart surgery with CPB, the use of intraoperative and post- operative theophylline may be effective in reducing postoperative inflammation, as demonstrated by lower white blood cell counts; but it does not significantly affect other clinical features.