INTRODUCTION: Mobile thrombus on pacemaker leads is a common phenomenon seen during intracardiac echocardiography in patients undergoing catheter ablation of atrial fibrillation (AF). PURPOSE: The purpose of our study is to assess the safety of RF ablation of atrial fibrillation in patients with mobile thrombus on pacemaker leads. METHODS: A total of 33 patients with AF and previously implanted cardiac pacemakers who had undergone radiofrequency pulmonary vein isolation since January 1st, 2015, were studied. All patients were anticoagulated with vitamin K antagonists (VKAs) or novel oral anticoagulants (NOACs), at least three weeks before the procedure. All procedures were guided by intracardiac echocardiography. Patients were divided into two groups: group 1 (15 patients with mobile thrombus on pacemaker leads) and group 2 (18 patients without mobile thrombus on device leads). RESULTS: The mean age of the pacemaker patients was 66,8 ± 7.7 years. The average thrombus length was 8.0 ± 3.3 mm. There were no differences in preprocedural anticoagulation with VKA or NOAC between groups (13% versus 17% for VKA, p = 1.0). Paroxysmal AF was less prevalent in the thrombus group (40% versus 83%, p = 0.01). There were no complications of AF ablation or arrythmia recurrence procedures in either group during the 30-day postoperative period. CONCLUSIONS: Radiofrequency ablation of AF can be safely performed in patients with mobile thrombus on pacemaker leads. Patients with persistent atrial fibrillation have an increased risk of pacemaker lead thrombus formation.