The patient manifested mild hemoptysis 2 days after the admission; however, other symptoms of respiratory failure were not observed. Although preoperative laboratory values showed mild anemia (hemoglobin, 9.5 g·dl Ϫ1 ; hematocrit, 29.8%) and thrombocytopenia (95 000 · mm Ϫ3 ), the patient's general condition was considered acceptable for right hip arthroplasty under general anesthesia to be undertaken. Prior to induction of anesthesia, a 22-gauge cannula was inserted into the left radial artery for monitoring of direct arterial blood pressure and arterial blood sampling. Although blood gas analysis performed just before induction of anesthesia showed hypoxemia (pH, 7.45; Pa O 2 , 48 mmHg; Pa CO 2 , 40 mmHg; base excess, ϩ4.9; Sa O 2 , 85% in room air), the patient did not manifest any other respiratory symptoms, such as tachypnea or dyspnea. The cardiovascular status was stable (arterial blood pressure, 132/ 60 mmHg; heart rate, 90 beats·min Ϫ1 ), and the possibility of congestive heart failure was excluded. Anesthesia was induced by thiopental 250 mg and fentanyl 100 µg i.v. in divided doses, and vecuronium bromide 4 mg i.v. was used to facilitate endotracheal intubation. Anesthesia was maintained by nitrous oxide (50%) with oxygen, sevoflurane (1.0%-1.5%), and intermittent administration of fentanyl 50 µg i.v. To exclude the possibility of bronchial obstruction, we performed a bronchofiberscopic examination immediately after the endotracheal intubation, but failed to find any significant obstruction. Only a small amount of bloody sputum was observed. During maintenance of anesthesia, the patient was hemodynamically stable, and her arterial oxygen tension was acceptable for the operation to be continued (FI O 2 , 0.5; pH, 7.41; Pa O 2 , 82 mmHg; Pa CO 2 , 45 mmHg; Sa O 2 , 96%; PET CO 2 , 5.2%) in the left lateral position. Immediately after the insertion of bone cement and a long-stem femoral component, Sp O 2 and PET CO 2 decreased to 60% and 3.6%, respectively, without significant changes in blood pressure and waveform on the electrocardiogram. Even after the start of in-