To examine the prevalence, clinical significance, and problems in the diagnosis of myocardial infarct (MI) extension, 103 patients with acute MI were studied. Each patient underwent enzymatic infarct sizing in the initial 72 hours and then had quantitative CK-MB (myocardial isoenzyme of serum creatine kinase) analysis at 8-hour intervals over the remaining hospitalization. In addition, daily standard 12-lead ECGs and documentation of prolonged (greater than 15 minutes) resting ischemic chest pain were recorded. MI extension, by CK-MB methods, occurred in 32 (31%) of 103 patients at 5.9 +/- 0.3 days after initial infarction. ECG changes suggesting MI extension occurred in 14 (14%), but only six of these patients had extension by CK-MB. Similarly, recurrent chest pain following initial MI occurred in 28 (27%), but enzymatic extension was evident in only 11 of these patients. MI extension resulted in significantly greater early in-hospital mortality (16%) compared to those patients without MI extension (2.8%, p less than 0.05). Thus MI extension occurs commonly and may explain some early in-hospital deaths post MI. The usual clinical and ECG diagnostic parameters utilized are insensitive indicators of enzymatic MI extension.