Seven hundred twenty-four consecutive patients admitted to a coronary care unit for chest pain underwent comparative evaluation of ECG and lactic dehydrogenase (LDH) and creatine kinase (CK) enzyme and isoenzyme patterns. Of the 724 patients, 419 (58%) had the myocardial component (MB) of CK detected; however, 69 (16%) of the latter had no abnormal elevation of total CK levels. This group with CK-MB but persistently normal total CK levels demonstrated fewer diagnostic QRS changes on ECG (17% vs 54%) and a lower incidence of LDH 1:2 inversion (28% vs 79%) than did the group with CK-MB and abnormally elevated total CK levels. However, no specific level of either total CK or CK-MB could segregate the patients with QRS or LDH level changes, which suggests that persistently normal levels of CK do not exclude the diagnosis of myocardial infarction. Evidence that myocardial necrosis can occur in the absence of an abnormal elevation of total CK levels is confirmed histologically in one patient. © 1982, American Medical Association. All rights reserved.