Sequential electrocardiograms for admission to 36 hours in 358 patient s with acute myocardial infarction (AMI) from the Pro-urokinase In Myocardial Infarction trial were assessed. The electrocardiogram was also examined at discharge in 69 of 358 patients. Patients underwent acute angiography, after which angioplasty was performed in most patients with impaired flow. The sum of the ST-segment deviation and Q- and R- wave voltages, and the QRS score were calculated and used for further evaluation. Development of Q waves, lost of R waves, and QRS score were completed within the first 9 hours after onset of AMI and remained stable thereafter. Reperfused patients had earlier stabilization and less severe electrocardiographic (ECG) abnormalities than nonreperfused patients. ST-segment elevation had already stabilized after 5 hours, was unchanged at 36 hours, and had significantly decreased at discharge. No significant ECG and clinical outcome differences were found between the Thrombolysis In Myocardial Infarction trial (TIMI) 2 and TIMI 3 patients. A 23.3% gain in ECG-estimated infarct size was found in the reperfusion group compared with a 12.0% gain in the nonreperfused group (p = 0.08). In summary, as early as 9 hours after onset of AMI, QRS changes were already complete. Thereafter, QRS morphology was stable. Thus, a QRS-based estimation of infarct size can be made as early as 9 hours after AMI. A similar ECG outcome for patients with TIMI 2 and 3 flow was found, which was significantly different from patients with TIMI 0 to 1 flow.