Carboxylesterase 1 (CES1) hydrolyzes the prodrug clopidogrel to an inactive carboxylic acid metabolite. We studied the pharmacokinetics and pharmacodynamics of 600 mg oral clopidogrel in healthy white volunteers, including 10 carriers and 12 noncarriers of CES1 c.428G>A (p.Gly143Glu, rs71647871) single nucleotide variation (SNV). Clopidogrel carboxylic acid to clopidogrel area under the plasma concentration-time curve from 0 hours to infinity (AUC0-∞ ) ratio was 53% less in CES1 c.428G>A carriers than in noncarriers (P = 0.009), indicating impaired hydrolysis of clopidogrel. Consequently, the AUC0-∞ of clopidogrel and its active metabolite were 123% (P = 0.004) and 67% (P = 0.009) larger in the c.428G>A carriers than in noncarriers. Consistent with these findings, the average inhibition of P2Y12 -mediated platelet aggregation 0-12 hours after clopidogrel intake was 19 percentage points higher in the c.428G>A carriers than in noncarriers (P = 0.036). In conclusion, the CES1 c.428G>A SNV increases clopidogrel active metabolite concentrations and antiplatelet effects by reducing clopidogrel hydrolysis to inactive metabolites.