Patients with a left atrial thrombus are considered as being at high risk for thromboembolic events. Although embolization may be asymptomatic, embolic events related to large thrombi are usually clinically manifest and tend to be associated with a worse prognosis. We describe the very unusual case of a patient with atrial fibrillation in whom two-dimensional echocardiography revealed a large, pedunculated, highly mobile left atrial thrombus attached by a thin stalk to the interatrial septum. Because of the high risk of embolism the patient was submitted to urgent surgery. At surgery, only the stalk was found. No mass was visualized at intraoperative transesophageal echocardiography. The patient had an uneventful postoperative course without signs of embolism. Computed tomography of the brain did not reveal any cerebral infarction.