Objective: The aim of this study is to evaluate the effects of single dose insulin, given prior to reperfusion, in patients undergoing coro- nary artery bypass surgery (CABG). Materials and Methods: One hundred and twenty patients were prospectively randomized to be given either insulin (Group 1;n=60), or saline (Group 2;n= 60). Blood samples were taken 15 minutes before the reperfusion and insulin was given (0.3 IU/kg) to the patients in Group 1. Arterial and coronary sinus blood samples were taken, after the release of aortic cross-clamp (0. min), and 5th -10th -15th minutes of reperfusion. Arterial and coronary sinus lactate and glucose levels, postoperative insulin, inotropic and intraaortic balloon pump requirements, need for defibrillation and postoperative dysrhythmia, creatine kinase- MB (CPK-MB) levels, and length of stay in intensive care unit (ICU) and hospital were compared. Results: In Group 1, arterial lactate levels were found to be lower at 0.min, coronary sinus lactate levels were found to be lower at 0-5-10th minutes of reperfusion compared to Group 2. Similarly, defibrillation, glucose, postoperative insulin and inotrop requirements, postoperative arrhythmia and length of ICU stay were lower in Group 1. The CPK-MB levels and length of hospital stay were similar in all patients. Conclusions: We conclude that single dose insulin given before the reperfusion period, has positive perioperative effects. Therefore it can be used in patients undergoing CABG surgery to decrease ischemia-reperfusion injury.