BACKGROUND: Resolution of left atrial thrombus after long-term oral anticoagulation enhances safe percutaneous transvenous mitral commissurotomy (PTMC); however, the short-term benefit has not been defined. Objectives: To estimate the resolution rate of left atrial thrombus among PTMC candidates after 6 months of oral anticoagulation and to determine its main predictors. DESIGN: Prospective cohort. SETTING: Community-based university medical center. PATIENTS: 219 PTMC candidates with thrombus demonstrated by multiplane transesophageal echocardiographic studies. MEASUREMENTS: The primary outcome was the status of the thrombus at the first 6-month follow-up; secondary measures were bleeding or thromboembolic complications. RESULTS: Among 219 PTMC candidates with left atrial thrombus (mean age [+/-SD], 39.6 +/- 7.4 years [range, 19 to 62 years]), complete resolution of thrombus, with an overall disappearance rate of 24.2% (95% CI, 18.5% to 29.9%), was demonstrated in 53 patients who subsequently underwent successful PTMC. In another 166 patients, the thrombus size was reduced by 24% (P < 0.001). No thrombus resolution was observed in the 27 patients with a left atrial body thrombus. Eighteen patients had minor bleeding. The significant predictors of thrombus resolution were a New York Heart Association class of 2 or less, a left atrial appendage thrombus size of 1.6 cm2 or less, a left atrial spontaneous echocardiographic contrast grade of 1 or less, and an international normalized ratio (INR) of at least 2.5. Patients with all of these predictors had a 94.4% chance of complete thrombus resolution (CI, 84.4% to 98.1%). CONCLUSIONS: After 6 months of oral anticoagulation, the left atrial thrombus disappeared in about a quarter of PTMC candidates so they could safely undergo PTMC. Less clinical severity, lower grading of the left atrial spontaneous echocardiographic contrast, a smaller thrombus, and a higher INR level predict thrombus resolution.