The purpose of this report is to document what we have observed in patients with familial abdominal aortic aneurysms (FAAAs) between 1987 and 1993. Patients with FAAAs were reviewed and compared with those without familial clustering with regard to age, sex, past history, laboratory data, smoking habits, and type of implanted graft. We identified 7 families among which a total of 15 members had AAAs. The incidence of familial clustering reached 5.4%. The mean age of the FAAA group was significantly younger than that of the non-FAAA group (mean age: 65.8 +/- 10.3 versus 71.0 +/- 7.3 years). In the FAAA group, furthermore, patients in the second generation were significantly younger than those in the first generation (mean age: 55.3 +/- 10.5 versus 69.6 +/- 7.4 years). FAAA was significantly more often complicated by ischemic cardiac diseases. There were no significant differences in other risk factors. Interestingly, however, we observed a morphological similarity in the shape of the aneurysms within each family. Histological examinations showed moderate or severe lymphocytic infiltration into the aortic adventitia in 6 out of 9 cases. FAAA is clinically important, because it can represent a high-risk group that may benefit from a screening program for early detection and elective management of AAA. Studies of FAAAs will be useful for elucidating the pathogenesis of AAAs.