The aims of this study were to determine the value of quantifying mitral valve disease by transoesophageal echocardiography before percutaneous mitral commissurotomy (PMC) and to analyse the incidence of embolic complications during PMC since the introduction of transoesophageal echocardiography. From March 1987 to December 1991, 317 patients with pure or dominant mitral stenosis were selected by Doppler echocardiography to undergo PMC at the Montreal Institute of Cardiology. The clinical features of the first 138 patients (Group I) were the same as those of the last 179 patients (Group 2) who also underwent routine transoesophageal echocardiography the day before the procedure. A thrombus in the left atrial appendage was observed in 8 patients in Group 2 (4.4%). No embolic complications have occurred since the protocol was changed to include routine transoesophageal echocardiography, whereas 4 embolic episodes, 3 of which were fatal, occurred in patients in group 1. The indication of PMC was turned down because of angiographically severe mitral regurgitation which was underestimated by transthoracic echocardiography in 2 patients in Group 1 (1.4%) and in 3 patients in group 2 (1.6%). The mobility, thickness and degree of calcification of the valves were attributed a score from 0-4 at transthoracic and transoesophageal echocardiography. No difference was observed in the scores of mobility (2.3 +/- 0.5 versus 2.3 +/- 0.05, NS) or valve thickness (2.1 +/- 0.4 versus 2.1 +/- 0.4, NS).(ABSTRACT TRUNCATED AT 250 WORDS)