Atrial fibrillation (AF) affects 4% of people over age 60, and it is also the most potent risk factor for stroke (relative risk of 5); 15% of all stroke, including >30% in those age >80, is attributable to AF. Seven randomized trials (RCTs) have shown anticoagulation can safely remove nearly all the risk of stroke due to AF. Nonetheless, only 1/3 of patients with AF are treated with anticoagulants, including only 19% of those age >80. Physicians are concerned about warfarin's safety in usual practice, particularly among the elderly, and about the burdens of monitoring anticoagulation. Concerns about risk of hemorrhage are supported by some observational studies of clinical practice, and by one RCT (SPAF II). As a result, several recent guidelines have suggested aspirin be used in the elderly, although evidence for its efficacy in AF is weak. The current application is designed to provide a rich source of information about anticoagulation for AF in 'real world' practice to optimally translate the efficacy seen in the RCTs into usual clinical care. It will also provide a resource for assessing genetic markers of risk of stroke in AF. The investigators will assemble two large cohorts: 1. the 8000 outpatients with AF in Kaiser Permanente of Northern CA; and 2. the 2500 patients managed by the Mass. General Hospital Anticoagulant Therapy Unit. Follow-up of these cohorts will address two important, linked topics, as follow: 1. specifying in clinical practice the rates of stroke and hemorrhage among patients with AF both on and off anticoagulants; and 2. identifying strategies to optimize control of anticoagulation. In detail, the research will provide the following: 1. precise and generalizable estimates of the rates of major bleeding of AF patients on warfarin in clinical care; 2. clinical risk factors for major bleeding, in particular assessing older age; 3. precise estimates of the rates of thromboembolism (TE) among AF patients not on anticoagulants; 4. clinical risk factors for TE with AF; 5. assessment of a novel marker of thrombosis risk -- the factor V Leiden gene mutation; 6. estimates of the optimal intensity of anticoagulation for AF; 7. identification of determinants of poor control of anticoagulation; 8. identification of correlates of the use of anticoagulation in AF; and 9. comparison of the quality of anticoagulation provided by anticoagulation units versus that of primary physicians. The investigators state that this research should lead to important health and economic benefits for older Americans.