One lung ventilation with a double lumen endobronchial tube during thoracic anesthesia is necessary for the protection of the healthy lung from contamination by the diseased lung and offers acceptable conditions for the surgeon. During one lung ventilation hypoxemia can frequently occur even with the administration of 100% oxyge. Recently we experienced markedly improved oxygenation during one lung anesthesia with a right sided double lumen endobronchial tube with application of CPAP 10cmH2O in the nondependent lung in a left lower lung brochiectatic patient. The healthy dependent right lung was ventilated by a conventional mechanical ventilator with FiO2 1.0, tidal volume 10ml/kg and respiration rate 12/min. Anesthesia was maintained with O2-halothane. Fifteen minutes after two lung ventilation in the right lateral decubitus position, the PaO2 and PACO2 were 400 and 33 torr respectively. 15 minutes after one lung ventilation PaO2 decreased to 99 torr. 15 minutes following application of CPAP 10cmH2O, the PaO2 increased to 229 torr, 30 minutes after CPAP 331 torr, and 2 hours after CPAP it rose to 373 torr. The nondependent lung was motionless and slightly expanded, and the surgical field was suitable for surgery.