Improvement in left ventricular ejection fraction is a measure of salvage of ischemic myocardium after reperfusion therapy for acute myocardial infarction. The degree of improvement in left ventricular ejection fraction may be influenced by many factors. Therefore, 137 patients in whom paired radionuclide angiograms were obtained within 24 h of acute infarction and before hospital discharge were retrospectively evaluated to determine which factors most affect improvement in ejection fraction. Only baseline ejection fraction correlated significantly with improvement in ejection fraction by both univariate analysis (ejection fraction as a continuous variable; p less than 0.001; ejection fraction as a categorical variable, less than or equal to 45% versus greater than 45%, p less than 0.0001) and multivariate analysis (p less than 0.0001). Reperfusion status (patent versus occluded infarct artery) and extent of coronary artery disease (one, two or three vessel) were significant factors by multivariate but not by univariate analysis. Location of infarction, treatment modality and time to treatment did not correlate with change in ejection fraction by either statistical technique. Thus, of those factors tested, baseline left ventricular ejection fraction is the most potent predictor of improvement in ventricular function after acute infarction. Knowledge of baseline ejection fraction may be helpful in deciding whether to treat some patients with equivocal indications or contraindications for reperfusion therapy. Clinical trials of reperfusion strategies should stratify patients on the basis of baseline ejection fraction if ejection fraction is to be used as an end point for myocardial salvage.