From family medical practices 15775 men and women aged 65-80 years were identified and randomized into two groups: one group was invited for ultrasonographic screening for abdominal aortic aneurysm (AAA), and the other acted as age- and sex-matched controls. Of the 7887 invited for screening 5394 (68.4 per cent) accepted. AAA was detected in 218 (4.0 per cent overall and 7.6 per cent of men). Aortic surgery was offered to the screened group if certain criteria were met and no patient died from rupture who was fit for operation and accepted elective treatment. The incidence of rupture was reduced by 55 per cent in men in the group invited for screening, compared with controls. The incidence of rupture in women was low in both groups.