To detect familial occurrence of abdominal aortic aneurysms (AAA), the siblings of patients with an AAA were screened by ultrasonography. 128 siblings of 32 patients operated on for AAA were invited. 56 brothers and 52 sisters accepted the invitation. An AAA was diagnosed in 16 brothers (28.6%) and in 3 sisters (5.8%). Six of these siblings were operated on because of an aneurysmal diameter 20 mm in excess of that of the rest of the abdominal aorta, the others were included in a follow-up programme. We conclude that the prevalence of the AAA among brothers is higher in comparison with previously known risk groups and that the family of patients with an AAA are to be considered for screening first.