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It is unclear whether atrial fibrillation/flutter (AF) newly diagnosed after ischemic stroke represents a preexisting risk factor that led to stroke, an arrhythmia triggered by poststroke autonomic dysfunction, or an incidental finding.We compared AF incidence after hospitalizations for ischemic stroke, hemorrhagic stroke, and nonstroke conditions using inpatient and outpatient claims between 2008 and 2015 from a nationally representative 5% sample of Medicare beneficiaries. We used validated International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes to identify AF-free patients hospitalized with ischemic or hemorrhagic stroke and matched them in a 1:1 ratio by age, sex, race, calendar year, vascular risk factors, and Charlson comorbidities. We then matched the combined stroke cohort in a 1:1 ratio to patients hospitalized for nonstroke diagnoses. We used survival statistics and Cox regression to compare postdischarge AF incidence among groups.We matched 2580 patients with ischemic stroke, 2580 with hemorrhagic stroke, and 5160 patients with other conditions. The annual postdischarge AF incidence was 3.4% (95% CI, 3.1%–3.7%) after ischemic stroke, 2.2% (95% CI, 1.9%–2.4%) after hemorrhagic stroke, and 2.9% (95% CI, 2.6%–3.1%) after nonstroke hospitalization. Ischemic stroke was associated with a somewhat higher risk of AF than hemorrhagic stroke (hazard ratio, 1.5; 95% CI, 1.3–1.8) or nonstroke conditions (hazard ratio, 1.2; 95% CI, 1.1–1.3). The latter association attenuated in sensitivity analyses limiting the outcome to AF diagnoses made by cardiologists (hazard ratio, 1.1; 95% CI, 0.8–1.5) or limiting the outcome to a minimum of 2 AF claims on separate dates (hazard ratio, 1.2; 95% CI, 1.0–1.5; P=0.09).New diagnoses of AF were more common after hospitalization for ischemic stroke than after hospitalization for hemorrhagic stroke or nonstroke conditions, but all hospitalized patients had a substantial incidence of new AF diagnoses after discharge and differences were attenuated when using more stringent definitions.