The ability to quantitate the amount of permanent left ventricular dysfunction in patients with unstable ischemic heart disease would have important clinical value. Left ventricular function curves were constructed in sixteen patients with acute myocardial infarctions and five patients with unstable angina pectoris syndrome at an average of 56 hours (±8) after the onset of symptoms. Fifty ml increments of low molecular weight dextran were rapidly infused into the right atrium during constant monitoring of the pulmonary artery end-diastolic pressure (PAEDP) via a Swan-Ganz thermodilution catheter. An average of 400 ml (range 200-800) was infused to produce a significant change in the PAEDP (range 3-13 mm Hg). The cardiac index was measured before and after the dextran infusion. The slope of the left ventricular function curve was calculated by dividing the change in the cardiac index by the change in the PAEDP. The sixteen patients with acute myocardial infarction underwent left heart catheterization and left ventricular biplane angiography an average of six months later. The five patients with unstable angina pectoris were studied within one month. The slope value of the left ventricular function curve was compared to angiographic ejection fraction by linear regression analysis and the correlation coefficient was 0.80. These data demonstrate that the slope of the left ventricular function curve in patients with acute myocardial infarction or unstable angina correlates well with the angiographically calculated ejection fraction; and as early as two days post myocardial infarction, the residual impirment of left ventricular function can be estimated.