Among patients scheduled for elective thoracic surgery at the Medical Center of Kyung Hee University, 15 patients undergoing lobectomy or pneumonectomy were selected. Three different ventilatory modes were employed and compared to the two-lung ventilation with 50% oxygen (control). First, patients were ventilated with 50% oxygen and left the unventilated lung to deflate during one lung ventilation (test 1). Second, continuous positive airway pressure (CPAP) of 10 cmH2O was applied to the ventilated lung while patients were under one lung ventilation with 50% oxygen (test 2). Lastly, patients were ventilated with 100% oxygen and unventilated lung was left to deflate during one lung ventilation (test 3). PaO2, A-aDO2 and Qsp/QT of three different ventilatory modes were observed and compared to that of control, and that of test 2 to test 1. The results were as followed: 1) Mean PaO2 in test 1 and test 2 were 98+/-24.0 mmHg and 126+/-34.8 mmHg, respectively and were significantly decresed as compared to the PaO2 of control, 234+/-21.4 mmHg. Comparing the PaO2 of test 1 and test 2, there was statistically significant increase in test 2 (P<0.01). 2) Comparing with A-aDO2 of control (68+/-22.5 mmHg), A-aDO2 in both test 1 and test 2 were significantly increased to 210+/-24.3 mmHg and 184+/-33.4 mmHg, respectively. there was significantly decreased in test 2 as compared to test 1 (P<0.01). 3) Shunt percentages (Qsp/QT) were measured as 8.3+/-2.3% in control, 25.4+/- 6.7% in test 1, 19.8+/-3.2% in test 2. Shunt percentages of test 1 and test 2 were increased significantly as compared to the control. Comparing the shunt percentages of test 1 and test 2, there was decreased in test 2 (P<0.01). Based on the above results, the application of appropriate CPAP to the unventilated lung during one lung ventilation is thought to be very effective in preventing hypoxemia. But, vigorous and meticulous monitoring, surveilance of patients and one lung ventilation with 100% oxygen are essential depending on the conditions of ventilated lung and long duration of one lung ventilation.