A 77-year-old woman underwent mitral valve replacement and tricuspid annuloplasty for severe mitral stenosis and tricuspid regurgitation with pulmonary hypertension. Two months later, the patient was readmitted because of marked edema. A new harsh pansystolic murmur was auscultated, and echocardiography revealed a jet from the left ventricle to the right atrium but no perivalvular leakage was detected at the mitral valve position. At operation, an 6mm defect adjacent to the tricuspid annulus in the interatrial septum and detachment of the anterior edge of the tricuspid ring were detected. The defect was closed using a pericardial patch. An inadequate stitch at the anteroseptal commissure in the previous operation led to left ventricular-right atrial communication.