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Background and Purpose: We assessed whether sinus bradycardia is an early sign of atrial dysfunction that might predispose to atrial thrombogenesis and therefore be associated with stroke risk.Methods: We retrospectively used inpatient and outpatient claims data from a 5% sample of Medicare beneficiaries ≥ 66 years old from 2008-2014. Our predictor variable was sinus bradycardia, defined as ICD-9-CM code 427.8x. Our primary outcome was ischemic stroke, ascertained using a previously validated diagnosis code algorithm. Patients with ischemic stroke or atrial fibrillation/flutter prior to or at the time of a bradycardia diagnosis were excluded. We used Cox regression analysis adjusted for demographics and vascular risk factors to evaluate the association between sinus bradycardia and the risk of stroke. Patients were censored at the time of a diagnosis of atrial fibrillation/flutter. In a confirmatory analysis, we assessed whether sinus bradycardia was more common in patients with cryptogenic stroke compared to stroke due to large-artery atherosclerosis or small-vessel disease using data from the Cornell AcutE Stroke Academic Registry (CAESAR), which comprises all adults with acute stroke at New York-Presbyterian Hospital/Weill Cornell Medical Center from 2011 to 2014.Results: Among 1,417,069 Medicare beneficiaries (mean age 73.4 +/- 7.6 years) who were followed for a mean of 4.36 (+/- 1.8) years, 63,385 patients received a diagnosis of sinus bradycardia. Bradycardia was significantly associated with stroke in unadjusted analysis (hazard ratio [HR], 1.7; 95% confidence interval [CI], 1.6-1.8) but not after adjustment for demographics and vascular risk factors (HR, 0.9; 95% CI, 0.8-0.9). Among 608 patients with cryptogenic or non-cardioembolic stroke in CAESAR, 93 (15.3%) had sinus bradycardia. We did not find an association between sinus bradycardia and cryptogenic stroke after adjustment for demographics, stroke severity, insular infarction, and vascular comorbidities (OR 0.9, 95% CI 0.4-1.9).Conclusion: We found no association between sinus bradycardia and future stroke risk in stroke-free patients nor an over-representation of sinus bradycardia among cryptogenic strokes versus non-cardioembolic strokes.