A 48-year-old man was admitted because of treatment-resistant, paroxysmal atrial fibrillation. Transthoracic, 2-dimensional echocardiography revealed a "mass" within the left atrium, thought to be a tumor. Transesophageal echocardiography showed it to be an atrial septal aneurysm. There is a relationship between atrial septal aneurysm and atrial supraventricular tachyarrhythmias and systemic emboli.