Ectopic beats triggering atrial fibrillation may arise from atrial myocardial extensions in the pulmonary veins, superior vena cava, coronary sinus, and vein of Marshall. We report two patients with paroxysmal atrial fibrillation initiated by ectopic beats originating from the inferior vena cava. The foci responsible for the ectopic beats were located and ablated, resulting in elimination of atrial fibrillation. Ectopic beats originating from the os of the inferior vena cava can result in atrial fibrillation. This finding may explain the recurrence of atrial fibrillation in some patients after pulmonary vein isolation.