Platelet aggregation is an important in vitro test to assess platelet aggregation response in IHD. The present prospective case control study was undertaken to evaluate the platelet aggregation response in IHD and the effects of aspirin therapy on it. Platelet aggregation was conducted on whole blood by the Chrono- Log whole blood Aggrometer model 540-VS. Various agonists used for platelet aggregation were collagen, ADP, Epinephrine and Thrombin. High platelet aggregation was observed in-patients of IHD as compared to controls by few or all of the reagents used. Platelet aggregation was high in both MI and angina as compared to control cases. However, cases of MI showed higher response than those of angina. Aspirin intake was associated with a decrease in platelet aggregation in patients of IHD. The platelet aggregation response was higher in PRP as compared to whole blood with similar concentration of reagents, however whole blood was equally effective as PRP in detecting hyper-responsive platelets in--patients of IHD.