To the author's knowledge, an intensive investigation of pulmonary function during the first 2 postoperative days has not previously been reported. Consequently, in this study the effect of regional surgery on respiratory function, monitored every 4 h during the first 2 postoperative days, was analyzed. Hypoxaemia was a constant finding in all patients undergoing general anaesthesia. The degree and duration of hypoxaemia and spirometry changes were related to the site of surgical incision. Clinical signs of atelectasis were commonly observed during the first postoperative day and occurred after the change in PO2 and spirometry. Atelectasis was more common in patients having upper abdominal surgery. The current investigation reveals that the earliest postoperative change indicative of atelectasis is the fall in PO2; that auscultatory changes do not always occur in the presence of postoperative hypoxaemia; that auscultatory signs are not indicative of the extent of the PO2 fall; and that radiology represents a crude method of assessing postoperative atelectasis.