Symptomatic thrombus formation due to a permanent pacemaker (PM) lead is a rare complication. It could be associated with serious outcome and should be suspected in patients who present with unexplained right heart failure, dyspnea, or syncope following dual-chamber PM implantation. A timely decision to perform an echocardiographic examination, followed by medical, thrombolytic, or surgical treatment can be necessary. We describe the case of an 84-year-old man who presented with syncope and hypotension a few days after PM implantation. A transesophageal echocardiography revealed a mobile mass in the right atrium attached to the pacemaker lead. Intravenous heparin allowed a complete resolution of the thrombus. K E Y W O R D S acute thrombosis, early thrombosis, pacing, pulmonary embolism, right atrium mass, right ventricular pacemaker lead 1 INTRODUCTION Thrombus formation around atrial and/or ventricular pacing leads is a rare but potentially severe complication following pacemaker (PM) implantation, which may present with pulmonary embolism. Venous thrombi usually have a typical highly mobile serpiginous shape on the transthoracic or transesophageal exams. They may obstruct tricuspid leaflets or more rarely cross a patent foramen ovale and cause paradoxical embolism. 1 The formation of thrombus usually requires predisposing risk factors such as blood stasis that occurs in atrial fibrillation as well as in some cardiomyopathies, or necessitate the presence of a foreign body such as a PM electrode. 2-4 A high clinical suspicion combined with early echocardiographic examination will help to diagnose this rare complication. Data on this rare phenomenon are limited to case reports.