Although two dimensional echocardiography can detect left ventricular thrombi In certain cardiovascular disease states, there Is theoretical concern that the acoustic Impedance properties of recently formed fresh thrombi may not allow their echocardiographic visualization. If such were the case, false negative studies might occur even with technically adequate echocardiographic examinations. To determine if the tissue acoustic properties of acute thrombi allow their visualization and differentiation from surrounding intracavitary blood and adjacent myocardium with two dimensional echocardiography, an in vivo canine model of acute left ventricular thrombus was studied. In 10 dogs left ventricular thrombus was induced using coronary ligation and subendocardial injection of a sclerosing agent, sodium rlclnoleate. Acoustically distinct left ventricular thrombi were imaged by two dimensional echocardiography within hours (mean ± standard deviation 121 ± 40 minutes, range 45 to 180), and the thrombi could easily be differentiated from surrounding blood and adjacent myocardium. Thrombi with a maximal dimension as small as 0.6 cm at autopsy were highly reflective and could be imaged with echocardiography. Histologic examination of the thrombi showed characteristic features of early thrombosis. In six dogs, echocardiographic imaging revealed two acoustically distinct areas of thrombi. Gross and microscopic examination of the thrombi in these animals confirmed two distinct types of thrombus with differing histologie features. Although technical aspects of the echocardiographic examination or certain biologic features of thrombi such as thrombus size may limit the detection of thrombi by echocardiography in certain situations, our data indicate that the tissue acoustic properties of recently formed thrombi are not a primary limitation to their echocardiographic detection. These findings support the use of two dimensional echocardiography in the investigation of the natural history, prevention and therapy of left ventricular thrombus in patients during the early course of acute myocardial Infarction.