OBJECTIVE Use of bilateral internal mammary artery (IMA) grafts during coronary artery revascularization procedures carries the potential for increased incidence of postoperative respiratory complications compared with use of unilateral IMA grafts. The purpose of this study was to compare the incidence of respiratory complications such as hypoxemia, atelectasis, pleural effusion, and diaphragmatic dysfunction in patients who received one or both IMAs as conduit grafts. METHODS Prospective, comparative study. METHODS Surgical ICU at a tertiary teaching hospital. METHODS Seventy-five patients with bilateral and 75 patients with unilateral IMA grafts. METHODS Serial postoperative PaO2/fraction of inspired oxygen measurements, radiographic scores of atelectasis and pleural effusion, duration of mechanical ventilation, length of ICU and hospital stay, and incidence of pneumothorax, pneumonia, and wound infection. RESULTS There was a higher incidence (51% vs 25%; p=0.002) and severity (0.48+/-0.09 vs 0.15+/-0.05 on the first postoperative day, 0.39+/-0.07 vs 0.27+/-0.07 on the fourth postoperative day, mean+/-SEM; p=0.004) of postoperative right lower lobe atelectasis in the group who received bilateral IMA grafts than in those who received left IMA grafts. This finding probably reflects the effects of additional surgical intervention on the right side of the chest. Incidence and severity of pleural effusion, gas exchange impairment, duration of mechanical ventilation, ICU and hospital stay, and incidence of pneumothorax, pneumonia, and wound infection were not influenced by use of bilateral IMA grafts (p>0.05). CONCLUSIONS We conclude that use of bilateral IMA grafts during coronary artery revascularization does not increase the incidence of postoperative respiratory complications compared with unilateral IMA grafting.