Background:Angina with normal coronary arteries, cardiac syndrome X, is a diagnosis of exclusion. The exact mechanism of this clinical syndrome remains unclear. Although the prognosis is as good as equal to that of normal population, symptoms related with the syndrome impair largely quality of life. Mean platelet volume showing the platelet size is an indicator of platelet function. Larger platelets are more active than smaller ones. Methods and results: We designed a study, evaluated mean platelet volume of the patients with cardiac syndrome X (group A) and stable angina (group B) and investigated the relation between groups. Eighty patients with cardiac syndrome X with a mean age of 51.08 +/- 9.79 years and 67 patients with stable angina with a mean age of 55.16 +/- 11.96 years were studied. At the end of the study, mean platelet volume of group A was significantly higher than that of group B, 10.55 +/- 1.08 fl vs. 9.39 +/- 0.58 fl, respectively (P < 0.001). Conclusion: Raised platelet size has been shown to be associated with adverse cardiac events. Mean platelet volume has increased in acute coronary syndromes and also in cardiac syndrome X in our study. Life style modification may optimize platelet size and improve symptoms in these patients.