14 years after a Björk-Shiley prosthesis had been implanted because of combined mitral failure, a 68-year-old woman complained of progressive decrease of physical powers combined with apnoea, retrosternal feeling of pressure and tachycardiac atrial fibrillation. Five weeks later, onset of cardiac decompensation with dyspnoea at rest and signs of congested pulmonary circulation were noted. Echocardiography revealed stenosis of the mitral valve prosthesis. The patient was operated on two days later. It became evident that the patency of the prosthesis was greatly obstructed by connective tissue which had proliferated at the ventricular side. The original valve prosthesis was replaced by a St. Jude bi-leaflet valve. The likelihood of prosthesis dysfunction must always be considered if there is a deterioration in the patient's condition after a long uncomplicated postoperative course following cardiac valve replacement.