A postoperative pseudoaneurysm is a rare complication that is difficult to diagnose. Computed tomography (CT) revealed a pseudoaneurysm in the ascending aorta of a 68-year-old female 8 days after mitral valve replacement and tricuspid annuloplasty. The defect was simply repaired during emergency surgery without cardiopulmonary bypass (CPB). The early detection of a postoperative pseudoaneurysm is important to avoid a second operation complicated by adhesions. Enhanced CT was useful for early detection in this patient. Pseudoaneurysm of the ascending aorta at the cannulation site can occur soon after surgery, and early recognition might allow simpler surgery without CPB.