We report a rare case of a patient diagnosed with mitral insufficiency grade III 12 years after mechanical aortic valve replacement. Transesophageal echocardiography described an eccentric mitral regurgitation-type systolic jet with color flow evidence of communication between left ventricle and atrium. Surgical intervention showed a circular defect in the mitral-aortic intervalvular fibrosa area, after removal of the mechanical valve, located beneath the noncoronary sinus causing the echocardiography-detected mitral insufficiency. A pericardial patch was trimmed to the appropriate size, and the defect was closed. The aortic valve was replaced by a stented pericardial bioprosthesis.