The authors studied the effect of insufflation of oxygen at 5 cmH2O into the nonventilated lung on the arterial oxygenation in the thoracotomy patients(n=14) who showed PaO2 below 100 mmHg or oxygen saturation below 95% during one-lung ventilation. When applying CPAP 5 cmH2O to the collapsed lung, we inflated the collapsed lung with reservoir bag that is one of the equipment of self made CPAP device. Hemodynamic parameter(mean arterial pressure and heart rate), arterial blood gas analysis and calculated shunt fraction was collected 10, 20, 30min after applying CPAP. PaO2 was significantly increased from 76+/-20mmHg to 180+/-66 (mean+/-SD) and shunt fraction was significantly decreased from 43.5+/-5.6% to 31.5+/-4.9% at CPAP 10 min. There was no significant difference in the PaO2 and shunt fraction between CPAP 10, 20 and 30 min. Heart rate was also significantly decreased with CPAP compared to pre-CPAP value. Other parameter(mean arterial pressure, PaCO2) was not changed with CPAP. The authors conclude that CPAP 5 cmH2O to the nonventilated lung is effective to improve oxygenation when hypoxemia occurs during one-lung ventilation.