The aim of this study was to assess the value of transoesophageal echo (TEE) in comparison with transthoracic echocardiography (TTE) in selecting candidates and evaluating the results of percutaneous mitral commissurotomy (PMC). One hundred and ten patients (pts) were examined by TEE and TTE before PMC. PMC was not performed in eight pts who had a thrombus in the left atrium detected by TEE but not by TTE. Out of the 102 other pts, TEE was better than TTE in detecting mild mitral regurgitation (MR) (84 vs 38, P less than 0.01). Spontaneous contrast was only shown by TEE in 70 pts. On the other hand, planimetry of the valvular area was only possible with TTE. This technique was also better in the assessment of the commissural area. During PMC, TEE enabled the interatrial septum to be punctured in two pts. After PMC, MR was trivial in 49 TEE cases compared with 36 by TTE and was moderate in 20 TEE pts compared with 12 by TTE (P less than 0.02). Transoesophageal colour Doppler showed a trivial atrial shunt in 63% of cases vs 13% by TTE (P less than 0.01).A small atrial septal defect was found in 30 cases only by TEE, and a spontaneous contrast persisted in all pts but six with moderate MR. In conclusion, TEE provides useful information in the ultrasonic assessment of PMC in particular with left atrial thrombi, mitral regurgitation, and the post PMC atrial septal defect. However, both methods are complementary and only TTE enables calculation of valve area.