Objectives-Although polymorphic variations in genes of the RAS system have previously been associated with susceptibility to AAA, such studies have been significantly limited by small sample sizes. This study was undertaken, using the largest case series yet reported, to determine whether common genetic variants of the RAS are associated with either susceptibility or severity of AAA. Methods and Results-The frequencies of 4 common genetic variants of genes related to the renin-angiotensin system were investigated in 3 geographically distinct, but ethnically similar, case-control cohorts, resulting in comparison of 1226 AAA cases with 1723 controls. In all 3 the AGTR1 1166C allele was significantly more common in AAA patients than controls (overall adjusted OR 1. Key Words: abdominal aortic aneurysm Ⅲ angiotensinogen Ⅲ angiotensin converting enzyme Ⅲ angiotensin Ⅲ Bradykinin Ⅲ genetic association Ⅲ renin T he renin-angiotensin system (RAS) is a potent mediator of cardiovascular homeostasis. The principal effector, angiotensin II (Ang II), is most widely associated with its role in the regulation of arterial blood pressure, but has been implicated in a wide range of nonpressor related actions including vascular remodeling, fibrosis, endothelial dysfunction, oxidative stress, 1,2 and dyslipidemia. 3 Reduction of Ang II levels, by inhibition of its converting enzyme (ACE), has been shown to significantly reduce cardiovascular disease morbidity and mortality. 4 Abdominal aortic aneurysm (AAA) is a common condition, being present in approximately 4% of white men between the ages of 50 to 79 years. 5 The condition is associated with a distinct familial component, with up to 20% of patients having one or more first affected relatives. 6 Evidence of a link between the RAS and abdominal aortic degeneration has been shown in experimental animal models, with exogenous Ang II inducing abdominal aortic aneurysms in the apolipoprotein E-deficient mouse, 7 and RAS blockade reducing spontaneous aortic elastic tissue degeneration in the rat, independent of changes in blood pressure. 8 In humans, ACE inhibitors have been suggested to reduce AAA rupture, an effect not observed with other antihypertensive agents. 9 Genetic polymorphisms of the RAS have been implicated as potential susceptibility candidates in a variety of vascular disorders including coronary artery disease, myocardial infarction, and hypertension. The role of the RAS related polymorphisms has been previously investigated in AAA patients. 10 -13 Fatini and coworkers, in the largest and most recent of these studies, investigated 250 cases and reported a significant association between ACE deletion homozygotes and AAA susceptibility. 10 All of the studies to date have involved small patient cohorts without adequate independent replication.Recently, a combinational effect of ACE, AGT, and AGTR1 SNPs has been reported for susceptibility to diabetic nephrop-