A free-floating ball thrombus in the left atrium is a rare complication of the mitral valvular disease. A 53-year-old man was admitted for pain and paresthesia on both legs. On admission he had auscultatory sign of mitral stenosis and mitral regurgitation, and the roentgenogram of his chest revealed a slight pulmonary venous congestion, enlargement of the pulmonary conus and cardiomegaly. Laboratory findings including complete blood counts, coagulation studies and blood chemistry were normal. An echocardiographic examination revealed a mitral stenosis and a free-floating ball thrombus in the left atrium. We performed the emergent open heart surgery for removal of the ball thrombus and mitral replacement successfully with Duromedics 29 mm valve. The size of thrombus was $39{ imes}32{ imes}30$ mm.