A 58-year-old man with fever and chest wall tenderness was seen 8 weeks after aortic valve replacement. His initial postoperative course had been complicated by mediastinitis, requiring antibiotics and surgical debridement. A transthoracic echocardiogram did not reveal the culprit lesion. Pseudoaneurysm of the ascending aorta was suspected, based on computerized tomographic and magnetic resonance images of the chest. Intraoperative transesophageal echocardiography confirmed the diagnosis of pseudoaneurysm and was a key component in the patient's operative management.