Digitised M mode echocardiograms were analysed in 22 patients with possible Björk-Shiley mitral prosthetic dysfunction. Patients with paraprosthetic mitral regurgitation had a significantly greater shortening fraction, an increased peak rate of dimension change during systole, and an increased peak velocity of circumferential fibre shortening than those with poor left ventricular function. Patients with a clotted prosthesis had lower values for shortening fraction and peak rate of dimension change during systole than patients with paraprosthetic regurgitation. In this latter group, the peak rate of dimension change during diastole and peak lengthening rate were greater than in either those patients with poor left ventricular function or those with a clotted prosthesis. In addition, the peak lengthening rate was greater in those with a clotted prosthesis than in those with poor left ventricular function. Thus M mode echocardiography is a useful method of assessing mitral prosthetic dysfunction and allows patients with paraprosthetic regurgitation to be distinguished from those with either poor left ventricular function or a clotted prosthesis.