Atrial fibrillation is the most common sustained arrhythmia reported in the United States; an estimated 1-2 million Americans have chronic nonvalvular atrial fibrillation. This disorder is associated with a substantial risk of stroke. Several recent studies provide evidence that anticoagulation therapy is indicated for stroke prevention in patients with nonvalvular atrial fibrillation after recovery from a minor stroke. Clinical and echocardiographic criteria help to identify those patients who are at especially high risk for thromboembolic stroke and are candidates for carefully controlled anticoagulation. In an effort to reduce the possibility of thromboembolic events following either chemical or electrical cardioversion, the American College of Chest Physicians has recently prepared guidelines for the use of anticoagulation in the conversion of atrial fibrillation. The efficacy of antiarrhythmic drug therapy for cardioversion is often difficult to assess. Furthermore, it is associated with major risks, including heart failure and exacerbation of arrhythmia, and minor risks, including systemic intolerance. A new National Institutes of Health trial, Atrial Fibrillation Follow-up Investigation of Rhythm Management (AFFIRM), will clarify the true risks and benefits of antiarrhythmic therapy for conversion of atrial fibrillation to sinus rhythm. Patients who cannot tolerate drug therapy may benefit from interruption of conduction in the bundle of His, followed by implantation of a permanent pacemaker, the use of radiofrequency energy ablation, or the implantation of an atrial defibrillator. Some patients may benefit from surgical procedures, such as left atrial isolation, the corridor operation, and the maze operation.