To assess the influence of percutaneous mitral commissurotomy (PMC) on left atrial spontaneous echo contrast of mitral stenosis, transesophageal echocardiography was performed before and 24 to 48 hours after the procedure, and on average, 6 months later in 82 patients. Fifty-nine patients (72%) were in stable sinus rhythm and 23 in permanent atrial fibrillation. Eleven patients (13%) had history of embolism, and 31 were on long-term anticoagulant therapy. The intensity of spontaneous contrast was graded as follows: 0 = no contrast; 1 = slight contrast; and 2 = intense contrast with the typical aspect of "smoke." PMC resulted in a twofold increase in the valve area irrespective of the method of evaluation used (2 cm2 after vs 1.05 before; p < 0.0001). Severe mitral regurgitation occurred in 3 patients who were operated on within 3 months after PMC. Left atrial spontaneous contrast was noted before the procedure in 53 patients (65%). Multivariate analysis showed left atrial size and cardiac index to be predictive factors of its presence (both p < 0.05). At early post-PMC investigation, the incidence of contrast was 50%, and at 6 months, only 28%. Sinus rhythm appeared to be the only independent predictive factor of the disappearance of contrast by multivariate analysis. In patients in atrial fibrillation, the prevalence of spontaneous contrast was 100% before PMC, 91% at early post-PMC investigation (p = NS), and 89% at the late study (p = NS); the rates were 51, 34 (p < 0.005) and 4% (p < 0.0001), respectively, in patients in sinus rhythm.(ABSTRACT TRUNCATED AT 250 WORDS)