Many reports have confirmed the value of transoesophageal echocardiography in the diagnosis of mechanical mitral valve prosthesis dysfunction: new biplane and multiplane probes seem to provide additional information in the assessment of cardiac disease. The aim of this study was to quantify the additional value of these new probes in the assessment of mitral valve prostheses. Seventy-five mitral valve prostheses were assessed with the multiplane probe, 45 normal bileaflet prostheses, 17 with regurgitant dysfunction and 12 with non obstructive thrombi and/or strands, and one with a blocked leaflet. In order to compare the respective values of monoplane, biplane and multiplane probes, the recordings were performed at 0 degrees, 90 degrees and from 0 degrees to 180 degrees in continuous sweep mode through the scanning plane. Globally, with the monoplane assessment, it was only possible to visualise both leaflets simultaneously in 13% of cases. The majority of prostheses was correctly analysed between 60 and 100 degrees. This was of paramount importance for the diagnosis of blockage of one leaflet. With respect to para-prosthetic valve regurgitation, the transverse view allowed visualisation of the lateral and paraseptal annular regions. The addition of a longitudinal view allowed visualisation of anterior and posterior regurgitant jets: the supplementary views provided by the multiplane probe allowed detection of small regurgitant jets in the diagonal planes between the longitudinal and transverse views. The multiplane probe offers the possibility of identifying the precise origin of the jet and helps quantification and peroperative localisation of its position. Small, non-obstructive thrombi and strands are better seen using a multiplane probe, especially when of small size. Therefore, multiplane transoesophageal echocardiography improves the assessment of mitral valve prostheses, the majority of diagnoses being, however, accessible with biplane probes.