The definition of acute myocardial infarction (AMI) is increasingly dependent on levels of biochemical markers, including troponin. We aimed to determine the levels of biochemical markers associated with definite evolutionary electrocardiographic (ECG) changes in patients with ST-segment elevation myocardial infarction. By examining the database of 855 patients from the troponin substudy of GUSTO-IIa, we selected patients with ST-segment elevation at baseline, evidence of evolution of the QRS, T, and ST-segment waveforms on the predischarge electrocardiogram, and 3 measurements of > or =1 of the following: creatine kinase (CK)-MB, troponin T, or troponin I. We identified 222 patients with evolutionary ECG changes. The median QRS score for this population was 5 points; the fifth percentile was 1. For patients with 3 CK-MB measurements, the fifth percentile as a multiple of the upper limit of normal was 2.1 (upper limit of normal 7.0 ng/ml). For patients with troponin T measurements, the fifth percentile as a multiple of the upper limit of normal was 11.0 (upper limit of normal 0.1 ng/ml). For patients with troponin I measurements, the fifth percentile as a multiple of the upper limit of normal was 3.8 (upper limit of normal 1.5 ng/ml). This study revealed that 95% of the patients with definite ECG evidence of AMI had a more than twofold increase in CK-MB and more than a 3- to 11-fold increase in troponin.