To avoid complications after percutaneous coronary intervention (PCI), such as acute and late thrombosis at the site of stenting and recurrent ischemic cardiovascular events, anti-platelet therapy is crucial . Several studies [2–4] have shown that dual anti-platelet therapy with aspirin and a thienopyridine ADP-receptor blocker is effective in preventing such acute and late complications. Clopidogrel is an ADP-receptor blocker that is used regularly in Japan. It has a better safety profile than ticlopidine, a previous-generation ADP-receptor blocker, but its main disadvantage is the wide interindividual variations of its anti-platelet effect . Hoshino et al. evaluated the anti-platelet effect of clopidogrel in Japanese patients and found wide interindividual variation as well as a proportion (approximately 14%) of clopidogrel non-responders .