The difference between the mortality rate from ruptured abdominal aortic aneurysm (overall mortality rate 85-95 per cent and operative mortality rate 23-63 per cent), and that for elective aneurysm repair (less than 5 per cent) is dramatic. Awareness of the existence of an abdominal aortic aneurysm is therefore essential. Of 1800 consecutive patients aged greater than or equal to 50 years referred for their first abdominal ultrasonography, 113 who had been referred specifically for suspected abdominal aortic aneurysm or vascular screening were excluded. The remaining 1687 patients (693 men and 994 women) form the study group. Apart from the symptom-directed examination, the entire abdomen of every patient was routinely studied by ultrasonography. The definition of an abdominal aortic aneurysm was a local dilatation of the aorta with an anteroposterior diameter greater than 30 mm or greater than 1.5 times the anteroposterior diameter of the proximal aorta. In 82 cases (4.9 per cent) an abdominal aortic aneurysm was disclosed; 61 were in men (8.8 per cent) and 21 were in women (2.1 per cent). The prevalence of abdominal aortic aneurysm as an incidental finding in men aged greater than or equal to 60 years was 11.4 per cent. In every patient aged greater than or equal to 50 years undergoing their first abdominal ultrasonography examination, the aorta should be screened for the presence of an aneurysm.