Between September 1990 and August 1991, 2291 men aged 65 years were invited for ultrasonographic screening of the aorta in the Gloucestershire aneurysm screening programme; 1748 (76.3 per cent) attended. An aortic diameter less than or equal to 2.5 cm was found in 1547 (88.5 per cent); 174 (10.0 per cent) had diameters in the range 2.6-4.0 cm, and 26 (1.5 per cent) had diameters greater than 4.0 cm. The mean(s.d.) aortic diameter was 2.1(0.55) cm and 97.5 per cent of patients had a diameter less than or equal to 3.3 cm. Any man over the age of 65 years with an aortic diameter greater than 3.3 cm has an aortic aneurysm.