The closing sounds of eleven St. Jude Medical cardiac prostheses at mitral position were analysed using the technique of the fast Fourier transform. The frequency-spectra contain a peak at about 1KHz, and decline the power to 5 to 7KHz. In two patients, 8.3 years and 4.5 years postoperatively, their frequency spectra of one leaflet showed a dip at 2.5KHz, respectively. The closing sound analysis is a useful methodd for the early detection of thrambosed SJM cardiac prosthesis at mitral position as well as at aortic or pulmonary position.