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. We report the case of a 55-year-old woman with a fenestrated atrial septal aneurysm that was incompletely visualized by transthoracic echocardiography. Subsequent transesophageal echocardiography demonstrated three defects within the atrial septal aneurysm with left-to-right shunting across each defect. Normal pulmonary venous connections were also defined. All echocardiographic findings were confirmed at surgery. This case demonstrates the additional diagnostic accuracy of transesophageal echocardiography for detecting disease of the atrial septum.