A retained surgical sponge (gossypiboma) is a rare but serious complication of surgery; most cases occur after intra-abdominal surgery. Intrathoracic gossypiboma is extremely rare, with only a handful of reported cases, most of which are associated with pulmonary surgery (1, 2, 3, 4, 5). Although almost all surgical sponges contain a radiopaque marker, usually a barium sulfate filament, detection of a retained sponge can be very difficult, particularly when its presence is not suspected. We present a case of anterior mediastinal gossypiboma following repair of subaortic stenosis in which the sponge marker was mistaken for a sternal suture wire on chest radiographs.