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Background: Atrial fibrillation (AF) is often newly diagnosed during heart-rhythm monitoring after ischemic stroke. It is unclear what proportion of these AF cases represent pre-existing disease and what proportion are triggered by stroke itself. Vascular brain injury that is either ischemic or hemorrhagic may be associated with subsequent AF.Objective: To compare the incidence of newly diagnosed AF after ischemic versus hemorrhagic stroke.Methods: We performed a retrospective cohort study using inpatient and outpatient claims data from a 5% sample of Medicare beneficiaries. We included all patients hospitalized with ischemic or hemorrhagic stroke (intracerebral or subarachnoid hemorrhage) from 2009-2014. Patients with any AF diagnosis before or during the index stroke hospitalization were excluded. The primary outcome was an incident diagnosis of AF. All variables were defined using previously validated ICD-9-CM diagnosis codes. Kaplan-Meier survival statistics were used to calculate cumulative rates of AF. Cox proportional hazards analysis was used to determine the association between stroke subtype and incident AF while adjusting for demographics, vascular risk factors, discharge disposition, and whether any post-discharge heart-rhythm monitoring was performed. We performed sensitivity analyses excluding patients with any post-discharge heart-rhythm monitoring and adjusting for the total number of post-stroke follow-up visits.Results: We identified 38,947 patients with ischemic stroke and 5,133 patients with hemorrhagic stroke. By 5 years, the cumulative rate of AF was 23.9% (95% confidence interval [CI], 23.2-24.7%) in patients with ischemic stroke versus 15.5% (95% CI, 13.8-17.3%) in those with hemorrhagic stroke. After adjusting for potentially confounding factors, patients with ischemic stroke were more likely to develop incident AF (hazard ratio, 1.3; 95% CI, 1.1-1.4). This association remained unchanged in our sensitivity analyses.Conclusions: New AF diagnoses are common after hemorrhagic stroke but even more common after ischemic stroke. Our findings suggest that AF found on heart-rhythm monitoring after ischemic stroke represents a mix of both pre-existing disease and stroke-induced dysrhythmia.