Transesophageal echocardiography (TEE) was conducted during mitral valve replacement (MVR) in six cases. Two-dimensional echocardiography demonstrated the prosthetic valve as an oval-shaped, echogenic ring and highly echogenic disc which showed rhythmical to-and-fro movement. Using two-dimensional color Doppler echocardiography (2D-CD-TEE), the regurgitant flow was demonstrated as the toward flow looked like a flame. 2D-CD-TEE made it possible to determine the regurgitant site from the origin of the flame. In most cases the regurgitant site was located just inside of the ring, thus suggesting intentionally designed transvalvular leak. In one case, however, the regurgitant site was just outside of the ring which suggested pathological paravalvular leak. But we could not find any evidence of mitral regurgitation on the left atrial pressure tracing or necropsy 20 days after the operation. 2D-CD-TEE is considered sensitive in detecting mitral regurgitation and an provide much information during the operation without interrupting the surgical procedures or interference by the operation. Additionally it can determine the pathological paravalvular leak from functional transvalvular leak.