Risk for stroke in patients with atrial fibrillation (AF) is highly heterogeneous. Increasing age, history of diabetes, hypertension, previous transient ischemic attack or stroke, and poor ventricular function are independent risk factors for stroke in patients with AF. Accordingly, some groups of patients with AF have low risk and some have high risk. In general, patients at high risk benefit most from anticoagulation therapy with warfarin. In general, if a patient is younger than 65 years of age and has none of the defined risk factors, the stroke rate without prophylaxis (aspirin or warfarin) is low. In patients 65 to 75 years of age with no risk factors, the risk for stroke is low with either aspirin or warfarin therapy; the choice is left to the caretaking physician. All patients older than 75 years and all patients of any age who have risk factors obtain striking benefit from the use of anticoagulation with warfarin. This benefit far outweighs any risk for major hemorrhage.