Atrial fibrillation (AF) is the most common arrhythmia in the world, with established public health implications due to increased morbidity, hospitalization, and health care costs.1 Both incidence and prevalence appear to be on the rise, with an annual US incidence of 3.7 per 1000 person-years in 2000.2 Sudden cardiac death (SCD) is the most devastating manifestation of cardiac ventricular electrical arrhythmias; more than 95% of individuals will experience an SCD within 10 minutes of presenting with ventricular tachycardia, ventricular fibrillation, pulseless electrical activity, or asystole.3 Despite advances in prevention and management of sudden cardiac arrest, 300 000 to 350 000 US lives are lost annually due to SCD, constituting approximately 50% of overall mortality from cardiovascular disease.4 As a consequence, the potential intersection of these two important conditions has long been of interest to investigators in either field. While an independent relationship between AF and overall mortality has been reported,5 an independent association between AF and the subgroup of patients with SCD had not been demonstrated.