We report the case of a large atrial septal aneurysm and a review of the literature. Atrial septal aneurysm is found in 1-8% of normal subjects. Its prevalence is higher among patients with ischemic stroke. Transesophageal echocardiography is an optimal tool for the diagnosis of atrial septal aneurysm. The clinical course may be complicated by arterial embolism, but mechanical complications may also occur, as in this case. Due to the lack of general agreement, treatment options should be discussed on an individual basis for patients with atrial septal aneurysm.