BACKGROUND AND OBJECTIVES: The reduction in the mean platelet component (MPC) concentration may be used to detect platelet activation. We performed this study to find the clinical usefulness of the MPC concentration in the differentiation of patients with stable angina from those with acute coronary syndrome (ACS). SUBJECTS AND METHOD: We evaluated 175 consecutive patients (57.9+/-10.4 years, 107 male) undergoing coronary angiography. The study patients were divided into two groups; patients with ACS (n=55, 57.5+/-11.8 years, 37 male) and those with stable angina (n=120, 58.1+/-9.7 years, 70 male). Venous blood samples were take into EDTA tube and immediately sent to laboratory room for measurement of the MPC concentration using the ADVIA(R) 120 hematology system. RESULTS: The MPC concentration was significantly decreased in those with ACS (27.3+/-1.2 g/dL vs. 28.6+/-0.9 g/dL, p=0.013) compared to those with stable angina, and also decreased in patients with unstable compared to stable angina (27.4+/-1.2 g/dL vs. 28.6+/-0.9 g/dL, p<0.001), but there was no difference in the MPC concentration between acute myocardial infarction and unstable angina (27.2+/-1.2 g/dL vs. 27.4+/-1.2 g/dL, p=0.939). An MPC concentration (28.05 g/dL demonstrated 74.5% sensitivity and 75.0% specificity in the differentiation of patients with ACS from the others in the Receiver Operating Curve analysis. The positive and negative predictive values were 51.6 and 86.5%, respectively, at that level. CONCLUSION: Measurement of the MPC concentration may be useful in the detection of ACS. Also, a decreased MPC concentration may be a very useful marker for the differentiation of unstable and stable angina.