Due to the high occurrence of cardiovascular diseases, clopidogrel is a frequently prescribed drug. As with any drug, it is important to find the optimal dose so that the patient is not underdosed and thus has an increased risk of developing side effects such as stroke and myocardial infarction. However, there can also be an overdose and thus the risk of bleeding. Because clopidogrel is a prodrug, it must be activated in order to exert its effect. One of the most important enzymes involved in activation is the CYP2C19 enzyme. This enzyme is important for more than 30 drugs, and its polymorphisms are associated with large interindividual variations in therapeutic response. Recent studies have shown a possible influence of the new haplotype CYP2C:TG on CYP2C19 enzyme activity and consequently clopidogrel activation. This study investigated the association of CYP2C19 (rs4244285 and rs12248560) and CYP2C (rs2860840 and rs 11188059) gene polymorphisms with the side effect of bleeding or ineffectiveness of clopidogrel. 145 subjects who received clopidogrel as part of therapy for various cardiovascular and cerebrovascular diseases were genotyped. Genotyping was done using the TaqMan® method. A statistically significant association between the allele without function (CYP2C19*2) and the ineffectiveness of clopidogrel was shown (p=0.043), which is in-line with several studies that have proven this. In contrast, we were unable to prove the association of the allele of accelerated CYP2C19*17 function and the new haplotype CYP2C:TG neither with ineffectiveness nor with the occurrence of bleeding. Further research is needed on a larger number of subjects in order investigate the association of the polymorphism of this enzyme and the CYP2C:TG haplotype with variability in clopidogrel therapeutic response.