The amount of blood lost from the circulation due to leaking abdominal aortic aneurysms was estimated in absolute figures by the difference in weight of the retroperitoneal tissues in 25 cases of sudden death and 25 controls matched for age, sex, height and weight. The proportion of total blood volume lost was calculated using established formulae. Eight subjects lost less than 500 ml or 10% of blood volume and only six lost more than 1500 ml or 25%. These figures suggest that most deaths are not due simply to haemorrhage and that death can occur after only a small loss of blood. There was no difference between subjects and controls in terms of heart weight, degree of coronary artery stenosis, and previous myocardial infarcts, nor was there any correlation of these with amount of blood lost. Endogenous cerebral opiates are known to switch off homeostatic cardiovascular responses to haemorrhage. This provides a possible explanation for circulatory collapse in patients with leaking abdominal aortic aneurysms who characteristically experience severe backache or abdominal pain.