The reported familial clustering of abdominal aortic aneurysm (AAA) indicates the possible rewards of family-based screening programmes with respect both to the number of asymptomatic aneurysms detected and to identifying associated genes. Ultrasonographic screening of 28 families (25 brothers and 28 sisters) was carried out together with collecting a history and a blood sample for analysis of the cholesterol level and genetic markers. Among the screened siblings six (11 per cent), all > 60 years old, had an AAA > or = 3.0 cm in diameter. A further 11 siblings (21 per cent), six of whom were < 60 years old, had a wide (2.5-2.9 cm) aorta. The presence of an aneurysmal or wide aorta was significantly associated with smoking (P = 0.027), male sex (P = 0.008) and a proband age of < 60 years (P = 0.031). Polymorphic genetic markers for type III collagen and haptoglobin were not informative in these families. These results indicate that the efficiency of screening siblings of patients with AAA could be improved by limiting it to brothers with a smoking history and/or siblings of younger patients. The familial component appears to be greatest in these younger patients.