peer reviewed ; PURPOSE: Familial clustering of abdominal aortic aneurysm was first noticed in 1977. METHODS: Through questionnaire and phone inquiry, familial data on 324 probands with abdominal aortic aneurysms allowed the establishment of 313 multigenerational pedigrees including 39 with multiple affected patients. RESULTS: There were 276 sporadic cases (264 men, 12 women); 81 cases belonged to multiplex pedigrees (76 men; 5 women). We compared familial and sporadic male cases; the ages at diagnosis were 64.1 +/- 7.9 years and 66.0 +/- 7.3 years (p < 0.05), respectively, the ages at rupture were 65.4 +/- 6.6 years and 75.2 +/- 8.6 years (p < 0.001), and the rupture rate was 32.4% and 8.7% (p < 0.001). Survival curves were computed. Relative risk for male siblings of a male patient was 18. We performed a segregation analysis with the mixed model, the most likely explanation for occurrence of abdominal aortic aneurysm in our families was a single gene effect showing dominant inheritance. The frequency of the morbid allele was 1:250, and its age-related penetrance was not higher than 0.4. CONCLUSION: This analysis indicates the preeminence of genetic factors on multifactorial/environmental effects of the pathogenesis of abdominal aortic aneurysm.