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Introduction: It is uncertain whether aortic diseases such as aneurysm and dissection are associated with aneurysmal subarachnoid hemorrhage (SAH).Methods: We performed a retrospective cohort study using 2008-2015 data from a nationally representative 5% sample of Medicare beneficiaries. Our exposure of interest was hospital discharge diagnosis of an unruptured or ruptured aortic aneurysm or aortic dissection. Our primary outcome was nontraumatic SAH. Predictors and the outcome were ascertained by validated International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) diagnosis codes. Survival statistics were used to calculate incidence rates. Cox proportional hazards analysis was used to examine the association between aortic aneurysm/dissection and SAH while adjusting for demographics, vascular risk factors, and Charlson comorbidities.Results: Among 1,781,917 beneficiaries, 32,551 (1.8%) had a documented aortic aneurysm or dissection. During 4.6 (±2.2) years of follow-up, 2,538 patients (0.14%) developed a nontraumatic SAH. The incidence of SAH was 0.09% (95% confidence interval [CI], 0.07-0.11%) per year in patients with aortic aneurysm/dissection as compared to 0.03% (95% CI, 0.03-0.03%) per year in patients without aortic aneurysm/dissection. After adjustment for demographics, stroke risk factors, and Charlson comorbidities, patients with aortic aneurysm/dissection faced an increased risk of SAH (hazard ratio, 1.9; 95% CI, 1.4-2.7).Conclusion: In a nationally representative sample of Medicare beneficiaries, aortic aneurysm/dissection was associated with an increased risk of nontraumatic SAH. This association may reflect shared vascular risk factors and/or a common genetic basis or underlying structural deficits within the arterial wall resulting in aneurysm formation.