A 39-year-old woman underwent aortic valve replacement (AVR) with a 21 mm St Jude Medical prosthesis and mitral valve replacement (MVR) with a 27/29 mm On-X valve prosthesis when she was 38-year-old, and she was discharged uneventfully. Five months after the operation, she was admitted with aggravated dyspnea. Upon admission she went into serious heart failure, followed by cardiogenic shock. Cineradiography showed a restricted opening of the On-X mitral valve prosthesis and transesophageal echocardiography demonstrated thrombus formation on the mitral valve annulus. We diagnosed thrombosed valve and reoperated urgently. Intraoperatively we found a large amount of fresh thrombus extending from the sewing cuff that restricted valve motion. The valve was cleaned and left in place. Although the heart recovered well, she lost some neurologic functions and was transferred to another hospital for rehabilitation. We also investigated the opening angle of the On-X mitral prosthesis in other patients who had clinically normal valve function.