Unexpected deaths still occur following major surgical procedures. The cause is often unknown but may be cardiac or thromboembolic in nature. Postoperative ischaemia, infarction and sudden cardiac death may be triggered by episodic or constant arterial hypoxaemia, which increases during the night. This study examined the circadian variation of sudden unexpected death following abdominal surgery between 1985 and 1989 inclusive. Deaths were divided into those occurring during the day (08.00-16.00 hours), evening (16.00-24.00 hours) and night (24.00-08.00 hours). Twenty-three deaths were considered to have been totally unexpected. Of 16 such patients undergoing autopsy, pulmonary embolism was the cause of death in five. In the remaining 11 patients, death occurred at night in eight (P