Nonrheumatic atrial fibrillation (AFib) is the most potent common risk factor for stroke, raising the risk of stroke 5-fold. Six randomized trials of anticoagulation in AFib consistently demonstrated a reduction in the risk of stroke by about two-thirds. In these trials, anticoagulation in AFib was quite safe. In contrast, randomized trials indicate that aspirin confers only a small reduction in risk of stroke, at best. Pooled data from the first set of randomized trials indicate that prior stroke, hypertension, diabetes, and increasing age are independent risk factors for future stroke with AFib. Individuals < 65 years old with none of the other risk factors might safely avoid anticoagulation; for all others, anticoagulation seems indicated. Studies of hemorrhagic risk highlight the importance of keeping the international normalized ratio (INR) < 4.0. Recent analyses also reveal that risk of ischemic stroke in AFib increases greatly at INR levels < 2.0. Efficacy and safety of anticoagulation in AFib depend on maintaining the INR between 2.0-3.0. Cost-effectiveness studies indicate that anticoagulation for AFib is among the most efficient preventive interventions in adults. Importantly, the benefits of anticoagulation in AFib accrue immediately. The implications for managed care organizations are that anticoagulation for AFib should be encouraged in their covered populations, and that dedicated anticoagulation services should be developed to promote system-wide control of anticoagulation intensity. Quality measures would include the proportion of patients with AFib who are anticoagulated, and the percentage of time patients' INR levels are between 2.0-3.0. Managed care organizations can benefit from recent research on anticoagulation for AFib; they have a responsibility to support future research and development efforts.