Patients after percutaneous coronary intervention (PCI) with stent implantation have to be treated by dual antiplatelet therapy, which standard part represents clopidogrel. The response to clopidogrel in population exhibits wide interindividual variability. According to some recent works, patients with clopidogrel resistance are in higher risk for recurrence of myocardial infarction in comparison with the patients with sufficient clopidogrel effectiveness. The treatment of clopidogrel resistance is still unknown. Our project should contribute to the better understanding of the clinical impact of clopidogrel resistance and its genetical determination. We will test the hypothesis, whether the clopidogrel resistance could be influenced by higher dose of this drug or by replacement to ticlopidine (ADP antagonist with different biotransformation in the liver). Therefore, 500 pts. will be tested to clopidogrel resistance. We expect 5-10% of resistent pts. This pts. will be treated by higher dose (150mg or 225mg/day) with repeated tests of clopidogrel effectiveness after each dose enhancement. If 225mg/day will be insufficient, clopidogrel will be replaced by ticlopidine with repeated test. We expect, that better definition of clinical and and genetic correlate of clopidogrel resistance will improve our knowledge of this disorder. Nevertheless, the achievement of sufficient effect of clopidogrel in some still resistant patients will lead to the improvement of the treatment