Hypotheses and Specific Aims: Atrial fibrillation is common and increases the risk of thromboembolic events. Current national guidelines recommend long-term treatment with warfarin, an oral anti-coagulant, for patients with atrial fibrillation and multiple thromboembolic risk factors. Adherence to warfarin is important because of its narrow therapeutic window, which requires frequent lab monitoring (i.e., INR) and dose adjustment. The majority of veterans on warfarin are managed in anti-coagulation clinics staffed by pharmacists who routinely contact patients to remind them to get blood draws, to re-enforce medication adherence, and to adjust warfarin dose based on INR values. Sub-therapeutic anti-coagulation increases the risk of thromboembolic events and is most commonly due to poor medication adherence.