Previous in vitro studies [K. J. Reynolds, Ph.D. thesis (1994); K. J. Reynolds and R. O. Stephen, J. Acoust. Soc. Am. 97, 60-68 (1995)] have shown that it is possible to detect minor mechanical changes to a prosthetic heart valve by acoustic analysis of the valve's closing sounds. This current study investigates the sounds produced by prosthetic valves implanted in patients. To reduce signal distortion at the thoracic surface, recordings were made with the patient submerged in water. Results show that stable reproducible averaged spectra can be obtained from implanted valves provided recording conditions are kept constant. The respiration cycle does not affect the recorded spectrum.