<b><i>Background/Aims:</i></b> One of the most common polymorphisms of ABCB1 gene, a synonymous mutation C3435T (rs1045642), is associated with increased in vivo activity. The main goal of this study was to determine the association of C3435T polymorphism with clopidogrel and 2-oxo-clopidogrel concentrations in plasma. <b><i>Methods:</i></b> The patients were recruited upon acute myocardial infarction diagnosis. They were all tested for ABCB1 C3435T polymorphism. In plasma, drawn 1 h after the drug administration, concentrations of clopidogrel and 2-oxo-clopidogrel were measured using UHPLC-DAD-MS analysis. <b><i>Results:</i></b> Due to differences in the maintenance doses, we have calculated the dose-adjusted concentrations of clopidogrel (0.2 ng/ml/mg (0.1-0.4)) and 2-oxo-clopidogrel (2.1 ng/ml/mg (0.5-4.6)). Patients carrying at least one C allele achieved significantly higher serum concentration of clopidogrel (p < 0.001), as well as dose-adjusted clopidogrel (p < 0.001) and 2-oxo-clopidogrel concentrations (p < 0.05). <b><i>Conclusion:</i></b> The ABCB1 3435CC genotype is associated with increased clopidogrel and 2-oxo-clopidogrel dose-adjusted concentrations. Therefore, the ABCB1 C3435T genotyping should be one of the parameters taken into account when deciding about the dosing regimen of clopidogrel.