We present herein a case of disc fracture of a Björk-Shiley valve prosthesis in the mitral position. A 69-year-old woman was admitted to our hospital with a sudden onset of dyspnea followed by deep shock. An echocardiography showed a severe degree of mitral regurgitation and moderate degree of tricuspid regurgitation. Forty-three years previously she had undergone a mitral valve replacement (MVR) for stenosis with the original version of a Björk-Shiley valve prosthesis in another institute. Emergency redo MVR was performed with a bioprosthesis and tricuspid annuloplasty with a semirigid ring. The disc of the extracted Björk-Shiley valve was found to have escaped from the metal housing with two intact struts. Although Björk-Shiley valve dysfunction due to Delrin disk abration has been rarely reported, complete disk fracuture is extremely rare. The important role of regular echocardiographic follow-up should be emphasized to prevent fatal valve fracture.