To evaluate the effect of late reperfusion of an infarct-related coronary artery on left ventricular (LV) function in the month after myocardial infarction (MI), findings from 386 patients in the Intravenous Streptokinase in Acute Myocardial Infarction (ISAM) trial were studied. All patients had a late peaking in the creatine kinase-MB serum time-activity curve, suggesting absence of early reperfusion. Significantly better LV function associated with patency of the infarct artery at angiography 1 month after the acute event confirmed the beneficial effect of reperfusion even if achieved beyond the time window for myocardial salvage. Thrombolysis commenced later than 4-6 h after symptom onset will be of benefit as long at it reopens infarct arteries more often and earlier than occurs spontaneously or from anticoagulation. The value of the size of the sum of ST-segment elevation (sigma ST increases) on admission ECG was evaluated from the total ISAM study population. The relation between sigma ST increases and final infarct size as well as mortality risk was highly significant. However, weak correlations largely limit the value of sigma ST increases to predict the outcome in an individual patient. Thus, whatever the relative beneficial effect in patients with smaller or larger MI may be, it would be unwarranted to withhold thrombolytic therapy from patients with smaller sigma ST increases on admission ECG.