Summary. Background: The paraoxonase activity of the enzyme paraoxonase-1 (PON-1) associated with high-density lipoprotein (HDL) may significantly influence clopidogrel’s antiplatelet and clinical efficacy as a result of its involvement in the clopidogrel biotransformation to the pharmacologically active thiol metabolite. We evaluated the possible relationships of HDL levels as well as PON-1 activities and the Q192R genotype with clopidogrel’s antiplatelet efficacy in acute coronary syndrome (ACS) patients. Methods and results: The platelet aggregation, P-selectin expression and platelet/leukocyte conjugates as well as the clopidogrel response variability (evaluated by the VASP phosphorylation test and expressed as platelet reactivity index, PRI) were assessed in 74 ACS patients undergoing percutaneous coronary intervention (PCI) in relation to the PON-1 Q192R genotype and to serum HDL-cholesterol levels, and PON-1 (paraoxonase and arylesterase) activities. Patients were loaded with 600 mg of clopidogrel followed by 75 mg per day. HDL-cholesterol levels and PON-1 activities at baseline (before clopidogrel loading) were not altered at 5- and 30-day post-clopidogrel loading, whereas baseline platelet activation parameters were significantly attenuated. At 5 days, 17 patients were clopidogrel non-responders (PRI: 64.2 ± 11.1%). HDL-cholesterol was inversely associated with platelet activation parameters independently on platelet response variability to clopidogrel whereas a negative association between platelet activation parameters and paraoxonase activity was observed in patients adequately responding to clopidogrel but not in clopidogrel non-responders. Similarly, the platelet activation markers were significantly higher in PON-1 Q192Q genotype carriers compared with those having one or two R alleles only in patients adequately responding to clopidogrel. Conclusions: PON-1 is an important determinant of clopidogrel antiplatelet efficacy only in patients adequately responding to clopidogrel. These findings may be clinically important in ACS patients receiving clopidogrel therapy, especially the first days after the episode.