Authors report the observation of a young woman of 30 years hospitalised for degeneration of a mitral biologic prosthesis. Transthoracic echocardiography visualizes a voluminous mass of the left auricle of which the character free and destitute of all parietal attachment, doesn't have can be established that by transoesophageal echocardiography that detects another thrombus also free and mobile of the left auricule, unrecognised by transthoracic way. The operative indication is carried in emergency but the patient dies suddenly some hours after this exam. From this observation and a revue of the literature, the contribution and risks possible transoesophageal echocardiography in the exploration of the mobile thrombi of the left auricle, are analysed. Account-held of the potential gravity of homodynamic and embolic complications, the emergency of the surgical indication is underlined.