In patients with mechanical mitral prosthesis, the presence of dysfunction and regurgitation of the prosthesis may be difficult to assess by standard precordial color flow Doppler. Moreover, the kind of mitral prosthesis regurgitant jet is often impossible to determine. We have recently studied 4 patients with clinically suspected mitral prosthesis dysfunction. In all of them the conventional transthoracic color flow technique was unable to evidentiate prosthesis regurgitation, whereas the transesophageal color flow Doppler assessed a partial displacement with a peri-prosthetic regurgitation in 3 patients, and a prosthetic endocarditis with intra-prosthetic regurgitation in 1. All studies were performed using an Aloka SSD 860 and 5 MHz transesophageal color Doppler transducer, using a topical anesthesia with 10% lidocaine. The procedure was well tolerated without any complication in all patients. Transesophageal color flow Doppler has specific improved capabilities over transthoracic conventional color flow Doppler and represents an important advance even for the noninvasive evaluation of patients with suspected mitral prosthesis regurgitation.