Aim. To study factors influencing platelet aggregation in patients with acute coronary syndrome (ACS). Subjects and methods. The investigation enrolled 147 patients with ACS. Their blood was sampled on days 1, 3-5, and 8-12 days after the onset of ACS. All the patients received acetylsalicylic acid (ASA) 300 mg on day 1, then 100 mg/day and clopidogrel 300-600 mg on day 1, then 75-150 mg/day. Platelet aggregation was analyzed in 65 patients on day 1 after ASA intake, but prior to clopidogrel therapy. The aggregation was induced by 5 and 20 µmol of ADP. Results. With the use of clopidogrel 75 mg/day on day 3-5, platelet aggregation was reduced by 2.1 and 1.7 times for 5 and 20 µmol of ADP, respectively, as compared to day 1 (ASA without clopidogrel) and remained unchanged on days 8-12. Increasing the dose of clopidogrel up to 150 mg/day potentiated its antiaggregatory effect. On day 1 (ASA without clopidogrel), there was a direct correlation between platelet aggregation levels and mean platelet volume (MPV) (correlation coefficients (r), 0.526 (p<0.001) and 0.368 (p=0.015) for 5 and 20 µmol of ADP, and between platelet aggregation levels and glycoprotein (GP) IIb-IIIa (r=0.387; p=0.002 and r=0.411 (p<0.001) for 5 and 20 µmol of ADP. No similar correlations were found on days 3-5 and 8-12 of administration of ASA and clopidogrel. The genetic polymorphism of GP IIb-IIIa (GP IIIa Leu33Pro) was not noted to affect platelet aggregation. Examining the effects of genetic variations in cytochrome P450 isoform CYP2C19 (a clopidogrel metabolizer) revealed the enhanced aggregation stimulated with 20 µmol of ADP in the carriers of slowly clopidogrel-metabolizing haplotype of CYP2C19 (differences were found on days 3-5 as compared to rapidly and routinely metabolizing haplotypes). Conclusion. In the patients with ACS, platelet aggregation is influenced by MPV, GP IIb-IIIa levels, and CYP2C19 polymorphism and is not by GP IIb-IIIa polymorphism.