Atrial fibrillation (AF) is a common arrhythmia that significantly increases the risk of stroke by the formation and embolism of left-atrial appendage thrombi. This risk can be substantially reduced with antithrombotic therapies such as aspirin or warfarin. Those with the highest risk receive the most benefit from adjusted-dose warfarin compared with aspirin or low-dose warfarin. Because of its efficacy in reducing strokes, adjusted-dose warfarin has been shown to be cost-effective in several different settings, but mostly for AF patients with at least 1 additional risk factor. Warfarin must be adjusted to international normalized ratios (INRs) within the target range of 2.0 to 3.0 to minimize the risk--as well as the cost--of stroke and bleeding. Subtherapeutic INR values occur commonly, but the consequences are increased risk of stroke and therefore increased costs. Of the several strategies available for managing anticoagulation, the key element to controlling costs is avoiding out-of-range values.