Inflammation is one of the major risk factors of acute myocardial infarction (AMI), and is associated with recruitment and activation of leukocytes. Myeloperoxidase (MPO), a highly abundant leukocyte enzyme, is secreted by leukocytes during inflammation. To evaluate the role of applicability of plasma MPO as a marker for the evaluation and prevention of AMI, we collected and studied clinical specimens from 40 AMI patients and 40 healthy control subjects. Plasma levels of MPO were determined by using a commercial kit (BIOXYTECH(superscript ®) MPO-EIA(superscript TM)). Leukocyte counts were determined by using an automatic cell counter. Medical records were reviewed and analyzed statistically. Levels of Troponin Ⅰ and CK-MB (both determined by automatic analyzers) from the AMI patients were determined. Our data showed that the average levels of MPO from the AMI patients were ten-fold higher than those of control group (149.7±103.7 ng/mL vs. 14.0±7.0 ng/mL). The average levels of Troponin Ⅰ and CK-MB in the patient group were higher than the normal values. Four AMI patients had normal levels of Troponin Ⅰ and CK-MB. However, their plasma MPO levels were all far above normal ranges, implying that MPO may be more useful as a marker. Analysis of the correlation between underlying diseases and cardiovascular diseases revealed that AMI patients had significantly higher proportions of hypertension and diabetes mellitus as composed to control subjects. In contrast, the percentage hyperlipidemia was significantly lower in AMI group. Significant correlation was also found between plasma levels of MPO and the respective leukocyte counts. Among the AMI patients, plasma MPO levels were significantly higher in patients with hyperlipidemia. In conclusion, the present study demonstrates the prognostic value of plasma MPO in the onset of AMI.