The main alternative therapeutic strategies for acute atrial fibrillation are rate versus rhythm control. A major concern in cardioversion of newly detected atrial fibrillation is the risk of thromboembolic events. The vast majority of these events occur in the first week following cardioversion. Transesophageal echocardiography has demonstrated that thrombus and dense spontaneous echo contrast may occur in the left atrium and left atrial appendage also in patients with acute atrial fibrillation (<48 hours) scheduled for cardioversion. Moreover, atrial function may become impaired immediately following successful cardioversion. Thus, the current North American and European guidelines recommend that patients with acute atrial fibrillation should undergo cardioversion under cover of unfractionated or low-molecular weight heparin followed by oral anticoagulation for at least 4 weeks in patients in patients at moderate-to-high risk for stroke. In line with the guidelines, new evidence from a large patient population suggests that after successful cardioversion of acute atrial fibrillation, patients have a low overall risk of thromboembolic events without any anticoagulation when they have no risk factors for thromboembolism. In contrast, the risk is in the range of 10% in patients with multiple classic risk factors for thromboembolism.