Atrial fibrillation is the most common rhythm disturbance and can also occur in absence of true cardiac disease. However, also in these cases, it can generate left atrial appendage thrombi with systemic embolic potential. A regular and well conducted anticoagulant therapy with dicoumarol derivatives, as indicated in these patients, is not always successful. We report the case of a patient with lone atrial fibrillation and a left atrial appendage thrombus resistant to anticoagulant therapy with warfarin, which disappeared after catheter ablation of atrial fibrillation by electrical disconnection of the pulmonary veins, restoration of sinus rhythm and dual-chamber pacemaker implantation.