We describe the very unusual case of a patient with a large, free-floating left-atrial thrombus secondary to severe mitral stenosis, in whom the peculiar symptoms and complications of a ball thrombus were absent. The patient's only symptom before the episode reported here was mild dyspnea, which was attributed to mitral stenosis. She experienced neither embolism nor syncope. While even her clinical signs did not indicate a left-atrial ball thrombus, both echocardiography and angiography showed a free-floating thrombus. Because of the risk of stroke and acute obstruction of the mitral valve, emergency surgery was performed upon diagnosis of the ball thrombus. The surgery, which consisted of removing the thrombus and replacing the mitral valve with a mechanical prosthesis, was uneventful. A computed tomographic brain scan prior to discharge did not detect any cerebral infarction.