Randomized clinical trials have demonstrated the efficacy and safety of oral anticoagulants in the prevention of systemic thromboembolism in nonrheumatic atrial fibrillation. The benefit of this treatment is particularly evident in patients in whom atrial fibrillation is associated with a major risk factor for systemic thromboembolism (patients > 75 years of age, history of hypertension, previous left ventricular failure or previous systemic thromboembolism) or those in whom two minor risk factors are present (patients between 65 and 75 years of age, diabetes, ischemic heart disease). According to these recommendations, all the patients > 75 years of age with chronic or paroxysmal atrial fibrillation should receive oral anticoagulant treatment to maintain an INR between 2.0 and 3.0. However, as the risk of bleeding during oral anticoagulant treatment increases with age, the benefit/risk ratio should always be evaluated in elderly patients. Although high risk patients do not benefit from aspirin treatment, aspirin or other antiplatelet agents might be indicated in medium risk patients or in those in whom the risk of bleeding with oral anticoagulants is considered too high. New antithrombotic regimens will be tested in the near future.