The purpose of the present study was to evaluate the correlations between high platelet reactivity (HPR) and the extent of coronary atherosclerosis and periprocedural myonecrosis in patients with acute coronary syndrome (ACS) who underwent percutaneous coronary intervention (PCI). A total of 485 patients who underwent PCI for ACS was studied. HPR was defined as ≥230 platelet reactivity units (PRU) in point-of-care P2Y12 tested by the VerifyNow assay. The incidence of multi-vessel disease (MVD) was higher in patients with HPR than those with no HPR (56.2% vs 45.8%, p=0.023). PRU values progressively increased with the number of diseased coronary arteries (1-vessel disease 221.8±86.7; 2-vessel disease 239.3±90.1; 3-vessel disease 243.4±84.5; p=0.038 by ANOVA). Multivariate analysis revealed that HPR was independently associated with MVD (Odds ratio 1.48, 95% confidence interval 1.01-2.25, p=0.048). Patients with periprocedural myonecrosis showed significantly higher PRU values compared with those without myonecrosis (258.6±94.5 vs. 228.5±85.6, p=0.013). Multivariate analysis revealed that HPR was an independent predictor for periprocedural myonecrosis as defined as any creatine kinase-myocardial band isoenzyme elevation or troponin T elevation. In conclusion, HPR is associated with MVD and periprocedural myonecrosis in patients with ACS and PCI. Thus, platelet reactivity after treatment with clopidogrel might be associated not only with blood clot formation but also with increased coronary atherosclerotic burden.