We report herein an uncommon clinical observation of a 82-year-old woman with paroxysmal postural dyspnea related to a giant ball-thrombus located in the left atrium and partly protruding through the mitral orifice. No mitral stenosis was otherwise disclosed. The patient had a previous medical history of chronic atrial fibrillation without any anticoagulant therapy. The atrial mass was easily removed and the postoperative course was uneventful. Disclosure of such a free-floating ball-thrombus in the left atrial cavity requires prompt surgical treatment because of high risks of acute hemodynamic decompensation due to obstruction of the left ventricular inflow or, more rarely, systemic embolic events.