Systolic time intervals (STI) were measured simultaneously with left ventricular pressure, right atrial pressure, and cardiac output in patients with acute myocardial infarction (AMI) within 24 hours of the onset of symptoms. Electromechanical systole (QS2) and left ventricular ejection time (LVET) were measured and preejection period (PEP) calculated and corrected for heart rate. LVET showed significant correlation with stroke volume (r = 0.62), confirming previous observations. Patients with significantly elevated left ventricular end-diastolic pressure (LVEDP) and clinical signs of congestive heart failure (CHF) usually exhibited a prolonged PEP when compared with normal subjects or patients with AMI who did not have signs of CHF. The normal PEP in the latter group might be due to adrenergic stimulation offsetting the effect of myocardial damage, although measurements of circulating catecholamines did not conclusively support this hypothesis. We conclude that STI are not reliable indices of left ventricular performance in AMI.