Real time two dimensional echocardiographic findings in four patients with a pathologically proved left ventricular pseudoaneurysm were compared with those in seven patients with a similarly proved true aneurysm of the left ventricle. Pseudoaneurysms produced a bounded echo-free space with a narrow neck that communicated with the left ventricular cavity. The maximal internal width of this neck (Omax) was much smaller than the maximal parallel internal diameter (Dmax) of the aneurysmal sac, and the ratio Omax/Dmax never exceeded 0.5. In all cases the pseudoaneurysm could be seen extending behind the intact portion of the involved left ventricular wall, and this finding produced a characteristic beak-like configuration in the studies of two patients. In contrast, true aneurysm resulted in local bulging and dilatation of the left ventricular wall so that the maximal internal width of the mouth (Omax) was nearly equal to, or actually represented, the maximal internal diameter (Dmax) of the aneurysm so that the ratio Omax/Dmax ranged from 0.9 to 1.0. Our preliminary study indicates that real time two dimensional echocardiography is useful in differentiating pseudoaneurysm from true aneurysm of the left ventricle. © 1980, All rights reserved.