A 63-year-old female was presented to emergency room with an abdominal pain. The patient had moderate mitral valve stenosis and atrial fibrillation. Abdominal computed tomography revealed right renal infarction. Transthoracic echocardiography showed a large mobile mass in the left atrium. Transesophageal two-and three-dimensional echocardiography showed a large mobile ovoid mass with a narrow stalk attached to the left atrial septum. It was thought to be a myxoma rather than thrombus. Anticoagulation with heparin was continued. When the operation was performed, there was no mass in the left atrium. It must be a thrombus and melt away.