The protective effects of high-dose ascorbic acid (250 mg/kg) on the myocardium were observed in 85 patients undergoing Cardiopulmonary Bypass (CPB). The changes in serum Malonyldialdehyde (MDA). Creatine Phosphokinase (CPK), Creatine Phosphokinase isozyme (CPK-MB) and Lactic Dehydrogenase (LDH) in group B (n = 45, receiving ascorbic acid) were lower (p < 0.05) than in group A (n = 40, no ascorbic acid) during and after CPB. The MDA remained at a higher level two days postoperatively; CPK and CPK-MB, the sensitive and specific reflectors of myocardial injury, recovered very slowly in the control group (A) after the operation. The hearts in all the patients of group B resuscitated automatically intraoperatively while five cases (12.5%) needed defibrillation in group A. The cardiac index (CI) measured in ICU in group B was higher than in group A (p < 0.05). The patients needed shorter ICU and hospital stays in group B than in group A. The results indicate that ascorbic acid can act as a scavenger of free radicals to decrease the peroxidation of the lipids present in the cell membrane and remove the radicals to protect the myocardium from ischemia-reperfusion injury effectively during and after open-heart operation.