We report a case of unexpected malfunction of prosthetic mitral leaflets, discovered by transesophageal echocardiography (TEE) immediately after implantation, due to selection of an inappropriate size of prosthesis; the problem was amended immediately by surgical revision. The subject was a 48-year-old man admitted for mitral valve replacement because of severe mitral stenosis. He had undergone mitral commissurotomy 21 years previously. Preoperative echo findings showed severe mitral stenosis with regurgitation and a huge thrombus in the left atrial chamber. During operation, the surgeon also discovered severe calcification over the mitral valve with rheumatic changes in both leaflets and the chordae. Because the posterior leaflet had totally fused with the annulus, resection could not be performed. A 33-mm Edwards-Carpentier porcine xenograft was then installed and sutured onto the approximate annular position. The posterior chordae were also not resected, and the atriotomy was closed. Once the aortic cross-clamp was removed and the patient was re-warmed, the left atrium distended rapidly. No ventricular wall motion was seen. Immediate TEE evaluation demonstrated that the prosthetic valve was dysfunctional and the leaflets were immobile. Total cardiopulmonary bypass was then resumed. A smaller 29-mm valve was reimplanted in the same anatomic position. The 33-mm tissue valve was then re-examined and checked for any structural abnormality or functional inadequacy. The prosthetic valve stent was intact and the leaflets were mobile upon gross inspection. The patient was successfully weaned from cardiopulmonary bypass with high dose inotropic agents and intra-aortic balloon pump support. In conclusion, TEE can provide us with immediate, accurate and detailed information to evaluate an unexpected circumstance and disclose an operative misadventure during or after cardiac surgery.