To assess the frequency, characteristics and relation to systemic embolization of mobile left ventricular thrombi, 154 patients with myocardial infarction were studied by cross-sectional echocardiography. In 5 (3%) a mobile left ventricular thrombus was detected. The electrocardiograms of all these patients showed anterior wall myocardial infarction pattern. One had cerebral embolism and two died suddenly. The cause of sudden death was not known, but it may be related to the presence of mobile left ventricular thrombi. In the remaining 149 patients, only 3 (2%) had cerebral or peripheral embolism. Mobile left ventricular thrombi was shown as rotating abnormal echoes arising from the left ventricular wall (infarcted area) by cross-sectional echocardiography. The size of thrombus was variable in each case. We concluded that cross-sectional echocardiography was useful in detecting mobile left ventricular thrombi, and that mobile left ventricular thrombi may be related to cerebral or peripheral emboli secondary to myocardial infraction.