OBJECTIVE Renin-angiotensin system (RAS) genetic polymorphisms are thought to play a role in cerebral aneurysm formation and rupture. The Cerebral Aneurysm Renin-Angiotensin System (CARAS) study prospectively evaluated common RAS polymorphisms and their relation to aneurysmal subarachnoid hemorrhage (aSAH). METHODS The CARAS study prospectively enrolled aSAH patients and controls at 2 academic centers in the United States. A blood sample was obtained from all patients for genetic evaluation and measurement of plasma angiotensinconverting enzyme (ACE) concentration. Common RAS polymorphisms were detected using 5' exonuclease (TaqMan) genotyping assays and restriction fragment length polymorphism analysis. RESULTS Two hundred forty-eight patients were screened, and 149 aSAH patients and 50 controls were available for analysis. There was a recessive effect of the C allele of the angiotensinogen (AGT) C/T single-nucleotide polymorphism (SNP) (OR 1.94, 95% CI 0.912-4.12, p = 0.0853) and a dominant effect of the G allele of the angiotensin II receptor Type 2 (AT2) G/A SNP (OR 2.11, 95% CI 0.972-4.57, p = 0.0590) on aSAH that did not reach statistical signifcance after adjustment for potential confounders. The ACE level was signifcantly lower in aSAH patients with the II genotype (17.6 ± 8.0 U/L) as compared with the ID (22.5 ± 12.1 U/L) and DD genotypes (26.6 ± 14.2 U/L) (p = 0.0195). CONCLUSIONS The AGT C/T and AT2 G/A polymorphisms were not signifcantly associated with aSAH after controlling for potential confounders. However, a strong trend was identifed for a dominant effect of the G allele of the AT2 G/A SNP. Downregulation of the local RAS may contribute to the formation of cerebral aneurysms and subsequent presentation with aSAH. Further studies are required to elucidate the relevant pathophysiology and its potential implication in treatment of patients with aSAH.