Coronary artery bypass grafting (CABG) improves the quality of life in patients with coronary artery disease (CAD), but complications may arise due to reperfusion syndrome, oxidative stress, and artificial circulation. This study aimed to investigate the impact of intraoperative hemolysis (IOH) on endothelial function in CABG patients. A total of 34 patients undergoing CABG with artificial circulation were evaluated for hemolysis levels and endothelial function using a reactive hyperemia test, both preoperatively and on the fifth postoperative day. Patients with higher hemolysis levels (0.6-0.8 g/L) exhibited a 28% worsening of endothelial dysfunction (p < 0.05), whereas those with lower hemolysis (0.1-0.2 g/L) showed only a 5% decline (p > 0.05). Significant differences between groups were observed. The study also found that the guanylate cyclase-mediated vasodilation mechanism was impaired in all patients. These findings suggest that CABG exacerbates endothelial dysfunction in proportion to IOH severity, likely due to oxidative stress and cytotoxic effects of red blood cell destruction products. Endothelial dysfunction may serve as a marker of reperfusion-related myocardial damage in CABG patients.