A 78-year-old cachectic woman who previously had repair of atrial septal defect was admitted to the hospital for congestive heart failure. Cardiac workup revealed chronic constrictive pericarditis; no evidence of coronary or valvular disease was found. She underwent corrective surgery for pericardiectomy. Intraoperative transesophageal echocardiography after pericardiectomy demonstrated acute development of severe mitral regurgitation, which was not preoperatively observed. She eventually required mitral valve replacement and tricuspid valve repair after conservative management failed. She recovered from the operation and was discharged. We believe that this is the first report of successful surgical management of mitral regurgitation that developed acutely after pericardiectomy.