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Background: Cardiac computed tomography (CT) is increasingly used to search for cardioembolic sources of acute ischemic stroke (AIS). We assessed the association between high-risk cardioembolic sources on cardiac CT and AIS., Methods: We performed a case-control study using data from a prospective cohort including consecutive adult patients with suspected stroke who underwent cardiac CT acquired during the initial stroke imaging protocol between 2018 and 2020. Cases were patients with a final diagnosis of AIS. Controls were patients with a stroke mimic (SMi). We excluded patients with a transient ischemic attack. Diagnoses were established by an adjudication committee. Cardiac radiologists assessed the presence of structural high-risk sources of cardioembolism according to predefined criteria. We used the Firth penalized likelihood method to perform a logistic regression, adjusted for age, sex, and history of myocardial infarction to determine the association between high-risk embolic sources and AIS. For the primary analysis, we excluded patients with a history of atrial fibrillation. In a secondary analysis, patients with known atrial fibrillation were included., Results: Of 774 patients, we excluded 167 patients due to no written informed consent or the diagnosis of transient ischemic attack. Of 607 patients, 107 patients had known atrial fibrillation and were excluded from the primary analysis. Of 500 included patients, 375 had AIS (75%, median age 70, 61% male) and 125 SMi (25%, median age 69, 42% male). A high-risk cardioembolic source was found on CT in 32/375 (8.5%) patients with AIS and 0/125 (0%) patients with SMi (adjusted odds ratio, 23.8 [95% CI, 3.3-3032.5]). Cardiac thrombi were the most commonly observed abnormality, present in 23 (6.1%) patients with AIS and 0 (0%) patients with SMi., Conclusions: A high-risk source of cardioembolism was detected on cardiac CT more frequently in patients with AIS than in patients with SMi. These data substantiate the clinical relevance of cardioembolic sources detected on acute cardiac CT in patients with ischemic stroke.