A free-floating thrombus in the left atrium is very rare in mitral stenosis. Such a thrombus can lead to sudden circulatory arrest and syncope or can cause severe cerebral or peripheral thromboembolic events. Clinical diagnosis is difficult, but left atrial thrombus should be suspected if patients with mitral stenosis and atrial fibrillation have intermittent or changing murmurs, emboli, or syncope. We describe the case of a patient with mild mitral stenosis under warfarin therapy, and a left atrial pedunculated thrombus discovered during the investigation for syncope attacks.