Phonocardiography is the graphic registration of various bands of the vibratory spectrum. We used the phonocardiogram as a monitoring apparatus. A 62-year-old man was scheduled for mitral valve replacement. After induction of general anesthesia and tracheal intubation, we inserted an esophageal stethoscope attached with a wireless microphone, amplifier, galvanometer and transcription system. The resulting phonocardiogram revealed a pan-systolic murmur. After successful replacement of the mitral valve, the phonocardiogram showed no sign of heart murmur. So we are convinced that the application of the phonocardiogram can be a useful monitoring device for valvular surgery.