The study includes two patients with obstruction of prosthetic heart valves by thrombosis. In the first patient, a thrombus attached to the ventricular side of the cage of a Smeloff-Cutter mitral prosthesis caused incomplete excursion of the ball in most cardiac cycles, which was detected by echocardiography. In the second patient a thrombosis of the atrial and ventricular side of a Lillehei-Kaster mitral prosthesis delayed opening of the disc. The initial part of the opening movement was "rounded", the excursion of the disk diminished. The cases presented indicate that in patients whose condition deteriorates after prosthetic valve replacement, echocardiography can help identify the cause. In particular, the technique makes it possible to differentiate between valvular dysfunction and muscular insufficiency of the left ventricle. Comparison with recordings obtained in the early postoperative period facilitate the detection of a malfunctioning prosthesis.