After mitral valve replacement, symptomatic deterioration and new murmurs characterstistic of atrioventricular valvular regurgitation are usually the result of periprosthetic leaks or prosthetic dysfunction with or without endocarditis. In the case which we are reporting, an iatrogenic fistula between the left ventricle and coronary sinus was responsible for the murmur and symptoms. This type of shunt has not been previously reported. Thorough debridement of the anulus is necessary during mitral valve replacement; additionally, previously implanted prostheses are occasionally embedded in the endocardial wall and must be excised. Hematoma in the atrioventricular groove and perforations of the posterior left ventricular wall are widely recognized complications of mitral valve replacement. Similar mechanisms of injury can cause other problems such as left ventricular-right atrial communications and the unique anatomic shunt described in this report.