We herein report a woman in her 70s was admitted to hospital with myocardial and multiple lacunar cerebral infarction, transthoracic echocardiogram revealed an active thrombus in the left atrium like a ping-pong ball. Anticoagulation was administered with low molecular weight heparin but did not work, the patient later developed cerebral infarction in the right frontotemporal lobe. There is a greater risk of severe cerebral infarction in active left atrial thrombus without mitral stenosis. Echocardiography played an important role in definitive diagnosis.