An analysis was made of the postoperative course of 158 cases of major surgery performed during 1971-1973. The operations comprised thoraco-abdominal resection of the oesophageal and cardia, abdominal duodenopancreatectomy, partial gastrectomy and enterectomy and partial hepatectomy. 63 per cent of the patients were given artificial respiration either prophylactically immediately after the operation (47 per cent) or at a later stage (53 per cent) on account of respiratory complications (arterial hypoxia below 55 mm Hg pO2). Postoperative mortality for the group of patients who did not have artificial respiration was 46 per cent, as compared with 32 per cent for the artifically ventilated patients - a difference of 14 per cent. The causes for this discrepancy, and especially the role of non-respiratory complications are discussed and the indications for prophylactic postoperative artificial respiration are reviewed.