In a group of 31 consecutive patients undergoing major abdominal surgery, severe postoperative arterial hypoxaemia was present in 10 cases (32%). The predictive value of the preoperative expiratory spirogram and the arterial PO2 have been examined. It is concluded that a history of chronic bronchitis may be of more value than either of these investigations in predicting the likely occurrence of postoperative hypoxaemia.