A 67-year-old man was admitted to a local hospital complaining of hemiparesis. Because of coexisting arrhythmia, he was examined by echocardiography and found to have an oval free-floating thrombus in the left atrium concomitant with mitral and aortic stenosis. He was transferred to our hospital for emergency surgery. He underwent an operation on the ninth day from the onset of neurological dysfunction, when a 2.5 x 2.5 x 3.0 cm ball thrombus was removed, and the diseased mitral and aortic valves were replaced. His postoperative course was uneventful, with no neurological sequelae. Though left atrial ball thrombus is rarely found in patients with mitral valve disease, when it is found, then immediate surgical intervention is recommended to avoid sudden death. However, there is a high risk that any cerebral lesion may worsen due to systemic heparinization. Therefore, the optimal time of surgery in a patient with a recent neurological deficit is controversial.