To assess the potential for atmospheric nitrogen to enter the nonventilated lung following the initiation of single-lung ventilation, the nonventilated lung of 10 patients undergoing video-assisted thoracoscopy was connected to the air in a water-filled spirometer, and gas movement out of and back into the lung was measured. Airway pressure from both lungs and pleural pressure from the nonventilated side were also measured. With each breath of positive-pressure ventilation to the ventilated lung prior to the thoracic cavity being opened to the atmosphere, the pressure transmitted to the opposite hemithorax generated a mean (range) tidal movement of gas in the nonventilated lung of 134 (65-265) ml. In addition, ongoing gas exchange resulted in a progressive influx of gas from the spirometer over the 110-120 s measurement period of a mean (range) volume of 155 (70-320) ml. This easily preventable influx of atmospheric nitrogen could, in theory, predispose to arterial desaturation and to delayed lung collapse after the parietal pleura is opened.