Dual antiplatelet therapy with aspirin and Clopidogrel for at least one year is essential in patients following an acute coronary syndrome (ACS) or percutaneous coronary intervention (PCI) with drug eluting stent(s. Interindividual variability in platelet response to clopidogrel has been reported, with several mechanisms being implicated for high post-clopidogrel treatment PR. High on-treatment platelet reactivity (HTPR) is associated with an increased risk of adverse events after PCI. Studies in patients on chronic clopidogrel treatment are scarce, mainly in diabetic patients where HTPR is frequently present and independently predictive of adverse events. Optimization of platelet inhibition in patients with HTPR by increasing clopidogrel or alternatively, by more potent P2Y12 inhibitors is a controversial issue, mostly studied in post PCI patients, while no data exist, to the best of the investigators knowledge, in stable patients on chronic clopidogrel treatment. Therefore all HTPR patients, that will accept to participate, will be enrolled will randomize (Day 0) in a 1:1 ratio to either clopidogrel 150 mg a day, or prasugrel 10 mg a day, until Day 14 post randomization. A 14 ± 2 day visit will be performed for PR measurement and safety evaluation, with the blood sample being will be obtained 16-18 hours after the last study-drug dose, will follow by crossover directly to the alternate therapy for an additional 14 days without an intervening washout period. At Day 28 ± 2 patients will return for the clinical and laboratory assessment as did on Day 14 visit.