The aim of this study was to elucidate the mechanism by which an atrial septal aneurysm is formed in the presence of an atrial septal defect by characterizing the distinguishing echocardiographic features of atrial septal defects between patients with and without an atrial septal aneurysm. The transesophageal echocardiograms of 30 consecutive patients who underwent surgical closure of a secundum atrial septal defect were compared with those of 8 normal controls. In patients with secundum atrial septal defect, the maximal diameter (mean ± SD) of the fossa ovalis was 21.73 ± 3.43 compared to 11.43 ± 1.00 mm in the control group (p < 0.01). In the 7 (23%) patients with atrial septal aneurysm, the mean maximal diameter of the fossa ovalis was 25.28 ± 3.03 compared to 20.65 ± 2.78 mm in those without an atrial septal aneurysm (p < 0.01). The atrial septal defect was smaller in patients with than in those without an atrial septal aneurysm. In 4 patients with atrial septal aneurysm who had a history of a cerebrovascular event, the interatrial communication was only detected by contrast echocardiography. In conclusion, in patients with atrial septal aneurysm, atrial septal defects tend to be smaller but the incidence of cerebrovascular events is greater.