OBJECTIVE Transesophageal (TEE) and Transthoracic (TTE) comparative echocardiographic analysis of patients with mitral valve prosthesis. METHODS Prospective study. METHODS In hospital and out patients with mitral prosthesis in a follow-up study by the Cardiology Department and referred to the echocardiographic laboratory of Gregorio Marañon General Hospital, Madrid. METHODS We studied 90 consecutive patients with mitral prosthesis diagnosis. METHODS Comparative and prospective echocardiographic study of transthoracic and transesophageal techniques in mitral prosthesis pathology. RESULTS TTE diagnosed 18% of dysfunctioning mitral prosthesis patients compared to 56% of cases identified by TEE. TTE observed 1% of mitral prosthesis vegetation compared to 8.8% by TEE. TEE diagnosed left atrial thrombosis in 2.2% and TEE in 10%. Left atrial spontaneous dynamic contrast was identified exclusively by TEE in 55% of all cases. Paravalvular mitral prosthesis leak was correctly identified exclusively by TEE in 20% of cases. CONCLUSIONS TEE has a greater diagnostic capacity compared to TTE in mitral prosthesis patients. This technique can give a greater security in evaluating mitral prosthesis thrombi, vegetations and leak, establishing a more precise diagnosis of mitral prosthesis dysfunction. The authors concluded that TEE is the technique of choice in noninvasive evaluation of patients admitted with the suspicion of mitral prosthesis dysfunction.