A 37-year-old woman had progressive shortness of breath and mitral stenosis was diagnosed. Despite the unusual finding of undegenerated septum primum on echocardiography and angiography, percutaneous transseptal mitral commissurotomy was successfully performed in this patient with rheumatic mitral stenosis under the guidance of online transesophageal echocardiography. (Circ J 2002; 66: 302 - 304)
