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To study the risk of thromboembolism in a nationwide cohort of atrial fibrillation patients undergoing direct current (DC) cardioversion with or without oral anticoagulant coverage.A retrospective study of 16 274 patients in Denmark discharged from hospital after a first-time DC cardioversion for atrial fibrillation between 2000 and 2008. Use of oral anticoagulant therapy within 90 days prior and 360 days after DC cardioversion was obtained from the Danish Register of Medicinal Product Statistics. The risk of thromboembolism was estimated by calculating incidence rates and by multivariable adjusted Cox proportional-hazard models. During the initial 30 days following discharge, the thromboembolic incidence rate was 10.33 per 100 patient-years for the no prior oral anticoagulant therapy group [n = 5084 (31.2%)], as compared with 4.00 per 100 patient-years for the prior oral anticoagulant therapy group [n = 11 190 (68.8%)], [hazard ratio associated with no prior oral anticoagulant therapy was 2.25; 95% confidence interval (CI), 1.43–3.53]. Thromboembolic risk stratification by the CHADS2 and CHA2DS2-VASc scores did not change the results. Hazard ratio with no oral anticoagulant therapy was 2.21; 95% CI, 0.79–6.77 and 2.40; 95% CI, 1.46–3.95 with CHA2DS2-VASc score 0–1 and CHA2DS2-VASc score 2 or more, respectively.Direct current cardioversion for atrial fibrillation without oral anticoagulation is associated with a high risk of thromboembolism. Notably, the risk is high in the initial period after cardioversion, indicating a hazardous association between DC cardioversion without anticoagulation and thromboembolism.