The study design will be a single center, prospective, observational pilot study. We will screen and enroll patients who are followed at Scripps Clinic and on chronic clopidogrel therapy. All patients will be eligible. A blood sample will be collected to perform DNA extraction and genotyping for the CYP2C19*2 polymorphism. Platelet function assays will be performed using VerifyNow point-of-care assay. We will recruit 60 patients with high platelet reactivity (PRU > 230): 30 wild-types (normal) and 30 carriers (heterozygous or homozygous) of the CYP2C19*2 polymorphism. These patients will be given 150 mg of clopidogrel for 1 week, at which time the platelet assay will be repeated. At that time, patients will be counseled by the study physicians regarding the potential risks and benefits of continuing to receive 150 mg of clopidogrel per day versus returning to the FDA approved dose of 75 mg daily.