Arterial blood gases were measured before and after operation in 14 patients undergoing conservative oesophageal surgery via a left thoracotomy. All the patients had a preoperative partial pressure of oxygen (PO2) of greater than 10 kPa, and none gave a history of chronic respiratory disease. All exhibited a fall in PO2 values after operation, the mean maximum reduction being 31%. The overall pattern of hhypoxaemia was similar to that previously reported after pulmonary resection, and upper abdominal surgery, characterised by the greatest reduction in PO2 on the first two postoperative days, followed by a gradual return towards preoperative values. In addition, there was a marked similarity in the degree of hypoxaemia observed by Parfey et al and by ourselves. These findings are attributed to similar changes occurring in pulmonary function in both series of patients, caused at least in part by different factors associated specifically with the two surgical approaches involved. In our experience, left thoracotomy, even without pulmonary resection, is associated with significant postoperative hypoxaemia. Knowledge of this may assist in the selection of patients for surgery and in their subsequent management.