Using two-dimensional echocardiography, the authors followed up 30 patients where in the course of treatment of acute myocardial infarction an intraventricular thrombus developed. A benign character (i. e. adjacent and immobile thrombus) was found in 21 patients, while a malignant (freely movable, floating, protruding) thrombus was recorded in 9 cases (70% as compared with 30%). All patients were followed up to the period of disappearance (regression) of the thrombus or within two years after infarction. The authors evaluated above all the ability of regression of thrombi and influencing their complication (above all systemic embolisms). In the whole group regression of thrombosis occurred in 25 patients (82%). In benign thrombi early regression--within three weeks after the development of infarction--occurred in 10 patients (33%) and late regression (more than three weeks) also in 10 patients. Analysis of the applied treatment revealed that all patients with early and nine patients with late regression of cardiac thrombosis had anticoagulation therapy till the thrombus disappeared. As to malignant thrombi, regression occurred only in five patients, always after at least three weeks. Systemic embolization (once into the cerebral arteries, twice into the arteries of the lower extremities) was recorded in three patients always with a malignant thrombus which was not affected by anticoagulant therapy. From the results of the investigation thus ensues that anticoagulation treatment during acute myocardial infarction and after it has in case of an intraventricular thrombus a favourable effect on the regression of the formed thrombus and it can reduce significantly the frequency of systemic embolisms which are the most serious complication.