The nature of intracardiac thrombi were studied, which were the clinical underlying conditions, relation to systemic embolism, growth of thrombus and effect of anticoagulant therapy on the size of the thrombi, in 818 patients with mitral value disease and 1000 patients with myocardial infarction by two-dimensional echocardiography. (1) Common underlying conditions were atrial fibrillation, enlarged left atrial cavity and predominance of mitral stenosis in cases with left atrial thrombi, and apical asynergy and low ejection fraction in cases with ventricular thrombi. The blood stasis should be the major factor in the formation of intracardiac thrombi. In a condition of blood stasis, dynamic intracavitary echoes which may represent erythrocyte aggregation were observed. (2) The incidence of systemic embolism in patients with thrombi was higher than that in patients without thrombi in cardiac disease. (3) The intracardiac thrombi were living. They grew and/or reduced their size spontaneously and sometimes became detached from the cardiac wall in the form of ball thrombi. (4) The effect of anticoagulant therapy on the regression of thrombi depends on its age.