Chronic non-rheumatic atrial fibrillation is associated with a risk of thromboembolic complications of about 5% per year. Previous myocardial infarction seems to be a significant risk factor for development of thromboembolic complications in chronic atrial fibrillation, whereas paroxysmal atrial fibrillation and isolated atrial fibrillation in younger patients may be associated with a lower risk of emboli. Silent cerebral infarction occurs more often in chronic atrial fibrillation than among controls in sinus rhythm. Three prospective trials of patients with atrial fibrillation found effect of warfarin on the occurrence of thromboembolic complications. In one study aspirin 325 mg daily was effective in patients below 75 years of age, but not in patients above this age. The other trials revealed no effect of aspirin.