Cardiovascular patients take acetylsalicylic acid (ASA) for preventing myocardial infarction and other thromboembolic complications. It is already known that in some patients this therapy is not effective. The aim of this study was to assess the percentage of ASA resistance on the sample of patients with coronary artery bypass grafting. Our study included 105 patients with coronary artery bypass grafting treated with ASA 150 mg/day or lesser. Platelet aggregation was measured by serum thromboxane B2 level as well as impedance aggregometry from whole blood to determine ASA antiaggregation effect. The percentage of ASA resistance was 41.9% with impedance aggregometry, and after determining the serum thromboxane B2 level this percentage was only 8.6%. The correlation between these two methods was weak (r = 0.443; P < 0.0001). Thromboembolic complications still occur in ASA-treated patients because some patients are resistant to ASA therapy. It would be useful to monitor the effectiveness of ASA therapy and give another antiaggregation drug to these patients to reduce adverse events. The problem is which test is ideal because different tests show different percentages of ASA resistance.