In two-dimensional echocardiographic studies of left ventricular thrombus in patients, an unusual pattern of dynamic left ventricular intracavitary echoes was identified in some hearts with severe apical dysfunction. These intracavitary echoes were noted in the apical region and were distinct from left ventricular thrombi. Certain features of the intracavitary echoes suggested that they were generated by regional stasis of blood. To study this phenomenon, echocardiography was performed in 11 dogs with experimental anteroapical infarction and associated left ventricular thrombus and in 6 dogs with infarction but no thrombus. The dynamic intracavitary echo pattern suggesting blood stasis was identified in the ischemic apex in dogs of both groups. These echoes had characteristics suggesting a fluid or semifluid state and could be distinguished from thrombi. In real time, the echoes moved in a slow, circular fashion at the apex and lacked well-defined borders; their configuration and acoustic intensity changed over short periods of time, and they could be rapidly altered by ectopic or mechanical contraction of the heart and by dopamine infusion. Postmortem examination showed that liquid blood produced the echoes. Additional studies demonstrated the echogenicity of static blood. Echocardiography of dog hearts with KCl-induced mechanical asystole showed the rapid development of diffuse echogenicity of the intraventricular contents; in vitro studies confirmed the echogenicity of static blood. These observations indicate that a spectrum of echocardiographic features characterizes ventricular blood under various conditions of flow and with frank thrombosis. The ability of echocardiography to detect in vivo stasis of blood in the left ventricle and to distinguish this from thrombosis has important clinical and investigational implications.