There is clearly a need for non-invasive techniques that will permit the study of the changes in left ventricular performance in the acutely ill patient early after an acute myocardial infarction as well as during the recovery period. Left ventricular ejection fraction as a measure of left ventricular performance can be obtained noninvasively by echocardiography.1/41) It is derived by measuring a single chord in the transverse plane of the left ventricular chamber. For the calculation of ejection fraction by this method two assumptions are made: (1) the geometry of the left ventricular chamber can be approximated by an elliptical model; and (2) the recorded transverse segmental function is representative of the entire left ventricle. The accuracy of this technique, therefore, might be limited when regional abnormalities in left ventricular wall motion are present, e.g., in patients with myocardial infarction. Alternatively, radioisotopic high-frequency left ventricular timeactivity dilution curves provide a means to measure left ventricular ejection fraction with high accuracy. (2,3,4) The calculation of the ejection fraction by this method is virtually free of geometric assumptions and, therefore, should be par-ticularly useful in patients with myocardial infarctions.