Little is known of the fate of left atrial thrombus (LAT) among candidates for percutaneous transvenous mitral commissurotomy (PTMC) after oral anticoagulation (OA) therapy. The aim of this study was to estimate the resolution rate of documented LAT, and to determine its significant predictors among this population. In this prospective cohort study all consecutive candidates for PTMC with documented LAT were followed between August 1996 and December 1999. All patients underwent both transthoracic (TTE) and transesophageal echocardiography (TEE) and received OA therapy (INR 2.0-3.0). The fate of LAT was studied serially using both TTE and TEE. A total of 75 patients was followed for six to 34 months (mean 12.2+/-6.2 months; total 914 patient-months). The LAT was completely resolved in 48 patients (5.25 per 100 pt-months; 95% CI 3.96-6.97). The overall resolution rate was 63.0 per 100 patients per year (95% CI 47.5-83.6). Patients with resolved LAT underwent uneventful PTMC. The median duration of LAT resolution was 16 months (95% CI 12.7-19.3). Significant predictors of LAT resolution were the size of the LAT (RR = 0.8 for each 1 cm2 increase in LAT; 95% CI 0.6-0.9; p = 0.012) and severity of the left atrial spontaneous echo contrast (LASEC) detected by TEE (RR = 0.6 for each 1 level increase of the LASEC severity score; 95% CI 0.4-0.9; p = 0.032). The smaller the LAT and the less severe the LASEC, the greater the likelihood of LAT resolution after OA therapy and the enhanced possibility of safely performing PTMC.