The use of dual antiplatelet therapy is considered standard of care in patients post percutaneous coronary intervention (PCI) with stenting. However, a significant proportion of patients is considered clopidogrel resistant and this resistance is shown to be accompanied by future adverse events. The aim of the study is to define in consecutive patients with acute coronary syndrome (ACS) undergoing PCI, those aged>75years and/or weighted<60 Kg with high on-clopidogrel platelet reactivity (Platelet Reactivity Units-PRU≥235) as estimated 24 hours post PCI with the VerifyNow assay. Those patients will be randomized after informed concent in 1:1 fashion to prasugrel 5 mg or clopidogrel 150mg daily. Platelet reactivity will be assessed at day 15 and then treatment crossover will be performed. At day 30 platelet reactivity will be determined as well.