The aim of this study are to establish the value of transesophageal echocardiography (TEE) in the diagnosis of mitral prostheses dysfunctions and to compare the performance of TEE and the transthoracic approach (TTE) in this context. From October 1990, the data when we started transesophageal ultrasound, till January 1992, we performed 218 TEE's. In 61 patients (pts) there was clinical suspicion of mitral prostheses dysfunction and TEE detected abnormal prostheses function in 22 pts (36%), 12 of which have been reoperated. In this group of 12 pts who underwent surgery, the basis for this study, the results of ETT and TEE were compared with surgeon's information. TEE confirmed the surgical findings in 11 patients (97%) with only one falsely positive (3%). On the other hand, TTE confirmed the surgical findings in only 6 pts (50%), confirmed partially in 3 (25%) and was falsely negative in 3 pts (25%). From these results the authors conclude: in the group of patients with mitral prostheses device dysfunction, that have been submitted to surgical treatment, there is great concordance between surgeon's findings and TEE information. TEE seems to be more accurate than TTE in the diagnosis of mitral prostheses dysfunction. Finally, TEE in conjunction with TTE is highly reliable in the diagnosis of mitral prostheses dysfunctions and spares patients from catheterism.