OBJECTIVE To derive and validate a clinical prediction rule that identifies patients after myocardial infarction who have preserved left ventricular systolic function. METHODS Retrospective analysis of a prospective cohort study, with a derivation set to generate a clinical prediction rule and a validation set to test the prediction rule. METHODS Urban tertiary care hospital. METHODS 314 consecutive patients admitted with myocardial infarction who had one or more of the following tests to determine left ventricular ejection fraction: transthoracic echocardiography, contrast left ventriculography, or radionuclide ventriculography. METHODS Left ventricular ejection fractions were determined by transthoracic echocardiography, contrast left ventriculography, and gated blood pool scan. RESULTS Multivariate analysis of patients in the derivation set yielded the following rule: The left ventricular ejection fraction is predicted to be 40% or more in patients who have 1) an interpretable electrocardiogram, 2) no previous Q-wave myocardial infarction, 3) no history of congestive heart failure, and 4) an index myocardial infarction that is not a Q-wave anterior infarction. In the derivation and the validation sets, the positive predictive value of the prediction rule was more than 0.98. CONCLUSIONS A simple clinical prediction rule using easily obtained historical and electrocardiographic data reliably identifies a substantial percentage of patients after myocardial infarction (40% in our hospital) who are likely to have preserved left ventricular systolic function. If validated in other patient populations, application of this prediction rule in clinical practice could result in a substantial decrease in the cost of treating uncomplicated myocardial infarction.