Abdominal aortic aneurysm (AAA) is a dilatation of the abdominal aorta which can, if undetected, lead to rupture. The mortality associated with ruptured AAA is estimated to be 90 percent, while elective repair has a mortality risk of approximately 6 percent. Ruptured AAA is a leading cause of death among older Americans. The identification of markers of AAA risk could lead to preventive intervention. AAA aggregates in families, and segregation analysis shows that familial risk of AAA is best explained by the segregation of a major gene with an autosomal recessive mode of inheritance.