Fifty-three patients were studied with combined echo-phonocardiography or phonocardiography alone following prosthetic valve replacement. In sixteen of these patients, clinical deterioration developed, and all subsequently underwent cardiac catheterization and/or surgery. Two patients came to autopsy. Included in this group of sixteen patients were five with obstructed prosthesis, six with paravalvular regurgitation, and five with left ventricular dysfunction. Measurements were made of the time interval from the aortic valve closure sound to the peak opening of the mitral prosthesis determined echocardiographically or to the mitral prosthetic opening click (A2-MVO). Echocardiographic studies of left ventricular wall motion were also performed. The A2-MVO interval was significantly shortened (P less than 0.01) with prosthetic valve obstruction (.05 +/- .02 sec) and paravalvular regurgitation (.05 +/- .01 sec) compared with normally functioning prostheses (Starr-Edwards ball valves .10 +/- .02 sec, Lillehei-Kaster tilting disc prostheses .09 +/- .01 sec). Shortening of this interval was not specific for these conditions because it was sometimes shortened with left ventricular dysfunction. Echocardiographic studies of left ventricular wall motion were helpful in distinguishing among prosthetic valve obstruction, paravalvular regurgitation and left ventricular dysfunction. The combined echo-phonocardiographic technique was especially helpful in detecting malfunction of tilting disc prostheses, because the technique enables measurement of the A2-MVO interval in the absence of an audible opening click.