Fifteen patients were studied with phonocardiography (phono) and echocardiography (echo) three to 13 months after mitral valve replacement with Lillehei-Kaster mitral valve prostheses. Echo measurements in all patients included prosthesis excursion (DE), opening velocity (OV), closing velocity (CV), and Q-to-mitral prosthesis closure (Q-MPC). In five patients, further, echo measurements included isometric contraction period (ICP), left ventricular ejection time (LVET), and time to completion of aortic valve opening (TCAO). Phono measurements in all patients included pre-ejection period (PEP), LVET, and PEP/LVET. Further phono measurements in the same five patients in whom further echo measurements were performed included Q to first heart sound (Q-S1), ICP, and isometric relaxation period (IRP). Phono and echo measurements were done sequentially. The echo results showed widespread values for DE, OV and CV with no correlation between clinical state, size of prosthesis, or postoperation duration. There was a positive correlation between CV and OV, r + 0.65. Q-MPC by echo averaged 72 msec, which is longer than reported values for Q-to-mitral valve closure in normal people. Similarly, ICP by echo was shorter than ICP by phono in every patient. Of the phono measurements IRP was shortest (45 msec) in one patient in congestive heart failure with a possible prosthesis malfunction. We conclude that echo- and phonocardiography are useful tools in evaluating mitral prosthesis function, but because normal values are widespread, individual measurements may be useful for followup in the same individual. Precise measurement of subdivisions of systolic intervals are now possible with these techniques.