Background: We conducted a retrospective study to compare two different techniques of internal mammarian artery (IMA) preparation concerning pleurotomy upon the effects of blood loss and pulmonary functions. Methods: Between January 1998 and November 2006, 1357 consecutive patients undergoing coronary artery bypass grafting (CABG) using the left IMA, either alone or in combination with saphenous vein graft, were included in this study. The patients were divided into two groups according to the pleural opening: Group I (n = 1046) patients underwent IMA harvesting with pleurotomy and Group 2 (n = 311) patients with intact pleura. Results: During the study, 27 hospital deaths (1.9%) occurred. The amount of postoperative blood loss and blood transfusion requirements were significantly higher in Group 1 than in Group 2 (p = 0.029 and p = 0.0001). The mechanical ventilation stay was significantly higher in Group 1 than in Group 2 (p = 0.0001). The incidence of left pleural effusion and atelectasis was significantly higher in Group 1 than in Group 2 on day 1 and day 3 after operation. Conclusions: These results demonstrate that preserving pleural integrity has beneficial effects on the postoperative blood loss. Postoperative blood loss and transfusion requirements were higher in patients with pleurotomy. Left pleural effusion, atelectasis, and mechanical ventilatory stay were significantly reduced in patients with preserved pleural integrity.