In this study, we aimed to determine systemic coagulation activity in patients with rheumatic mitral stenosis and to define determinants of a possible prethrombotic state. Peripheral venous plasma level of thrombin-antithrombin III complex was measured in 84 consecutive patients with rheumatic mitral stenosis who had no left atrial thrombus by transesophageal echocardiography. The patients had significantly higher thrombin-antithrombin III complex values (mean +/- SD = 9.6+/-15.9 ng/ ml) compared with the healthy subjects (2.1+/-1.8 ng/ml) (P<0.001). Among many clinical and echocardiographic variables, severe mitral regurgitation (odds ratio = 6.7, P<0.001) and left atrial spontaneous echo contrast (odds ratio = 22.8, P<0.001) appeared as significant predictors of the increased systemic coagulation activity in multivariate logistic regression analysis. In conclusion, systemic coagulation activity is increased in the patients with rheumatic mitral stenosis, and coexistence of severe mitral regurgitation and presence of left atrial spontaneous echo contrast are determinants of this increment.