Loss-of-function mutation of the gene encoding the CYP450 2C19 enzyme has emerged as a likely determinant of resistance to clopidogrel therapy. The primary hypothesis of the proposed research is that among patients with confirmed loss-of-function alleles of the CYP2C19 gene, increasing the maintenance clopidogrel dose from 75 to 150 mg will result in significant reduction in the rate of measured clopidogrel resistance defined by multiple measures of platelet function