The extent of initial acute myocardial infarction (AMI) and subsequent patient prognosis were studied using 2 independent indicators of AMI size. Two inexpensive, readily available techniques, the complete Selvester QRS score from the standard 12-lead electrocardiogram and the peak value of the isoenzyme MB of creatine kinase (CK-MB), were evaluated in 125 patients with initial AMI. The overall correlation between peak CK-MB and QRS score was fair (0.57), with marked difference according to anterior (0.72) or inferior (0.35) location. The prognostic capabilities of each measurement varied. Peak CK-MB provided significant information concerning hospital morbidity or early mortality (within 30 days) for both anterior (χ2 = 9.83) and inferior (χ2 = 9.83) AMI locations; however, the QRS score was significant only for anterior AMI (χ2 = 9.50. For total 24-month mortality, the QRS score alone provided the most information (χ2 = 10.0, p = 0.0016), which was not improved with the addition of CK-MB (χ2 = 0.07, p = 0.79). This study shows a good relation between these 2 independent estimates of AMI size for patients with anterior AMI location. Both QRS and CK-MB results are significantly related to early morbidity and mortality; however, only the QRS score is related to total 24-month prognosis. © 1986.