The prediction of left atrial thrombi based on clinical and investigative data was evaluated prospectively in 100 consecutive patients with significant mitral stenosis. Nineteen patients had left atrial thrombi by surgical findings. Age, atrial fibrillation, and mitral valve area were the variables that predicted the presence of left atrial thrombus, whereas sex, dimension of left atrium, history of systemic embolism, history of previous mitral valvuloplasty, and associated significant mitral regurgitation were not. Patients with atrial fibrillation have a sixfold increase in risk of atrial thrombi compared with patients in sinus rhythm. Transthoracic echocardiography detected 11 (58%) of 19 and transesophageal echocardiography detected 17 (89%) of 19 thrombi found by surgical inspection. Multivariate analysis showed that age and atrial fibrillation were the best predictors of left atrial thrombus. In general clinical practice, these variables could be used to predict left atrial thrombi in patients with mitral stenosis. However, in certain situations such as prior percutaneous balloon mitral valvuloplasty or prior electrical cardioversion, transesophageal echocardiographic examination should be used for high accuracy in the detection of extent and location of left atrial thrombi.