The diagnosis of atrial septal defect by transthoracic echocardiography remains difficult in a small subset of patients because of either suboptimal acoustic windows or unusual anatomy, for example, fenestrated defects. wer report the case of a 52-year-old woman with a fenestratdd atrial septal aneurysm that was incompletely visualized by transthoracic echocardiography. Subsequent transesophageal echocardiography demonstrated atrial septal defect and two fenestrations within the atrial septal aneurysm with left-to-right shunting. All echocardiographic findings were confirmed at surgery. This case demonstrates the addinional diagnostic accuracy of transesophageal echocardiography for detecting disease of the atrial septum.