Platelet flow cytometry is an emerging tool in diagnostic and therapeutic hematology. It is eminently suited to assess the expression of platelet surface receptors both qualitatively as well as quantitatively. It can thus serve as a useful parameter for in vivo expression of platelet activation and thus fore-warn the risk of thromboembolism which is inherent in patients with untreated type 2 diabetes mellitus. This technique can also be used to study and compare the effect of various antiplatelet drugs on the level of platelet activation and to establish any dose-effect relationship of these drugs. Topographical localization of platelet granules and study of platelet-platelet and platelet-leukocyte interaction is also possible by this procedure. All these parameters serve as pointers towards the presence of activated platelets in the circulation with its thromboembolic consequences. This study was undertaken with the primary object of determining platelet reactivity in patients with untreated type 2 diabetes mellitus and to study the effect of clopidogrel administration on the state of in vivo platelet activation in these patients.