Clopidogrel plus aspirin is a standard antiplatelet aggregation regimen in cardiovascular diseases, especially after implantation of a coronary stent. Interaction between clopidogrel and proton pump inhibitors theoretically reduces clopidogrel's antiaggregation effect, but the evidence is controversial. A total of 30 healthy subjects and 74 patients with a coronary stent were given a 300 mg loading dose of aspirin and 300 mg clopidogrel and then 100 mg aspirin/75 mg clopidogrel daily for 14 days. Subgroups were concomitantly treated or not treated with esomeprazole (20 mg/day). Clopidogrel significantly reduced adenosine diphosphate-induced platelet aggregation in healthy and stent-implanted subjects on days 7 and 14. Healthy subjects receiving esomeprazole showed a significantly higher platelet aggregation rate than those not receiving esomeprazole, but esomeprazole had no effect in patients with a stent. Aspirin plus clopidogrel did not result in significant gastrointestinal complications. These differential effects of esomeprazole on the antiplatelet activity of clopidogrel in healthy individuals and patients after coronary stent implantation merit further investigation.