Background Aspirin administered early after coronary artery bypass grafting surgery (CABG) improves graft patency and patients survival. However, the antiplatelet effect of aspirin seems to be variable and aspirin resistance is currently still being discussed. The aim of the study was to assess aspirin efficacy in the early postoperative period. Methods Forty patients undergoing elective CABG surgery (20 in on-pump and 20 in off-pump) were enrolled in the study. Functional and biochemical responses to aspirin were evaluated by arachidonic acid (ARA)-induced platelet aggregation and urine 11-dehydro Thromboxane B₂ metabolite excretion. Samples were collected before surgery (baseline; ≥7 days after aspirin withdrawal) and on days 1, 2 and 5 after surgery. Results Median baseline ARA aggregability was 55%. On day 1, platelet aggregability decreased (12%, P < 0.001). On day 2, despite the aspirin administration, platelet aggregability exceeded the values from day 1 (38%, P < 0.001). Only on day 5, sufficient inhibition of platelet aggregation was achieved (8%, P < 0.001). Median preoperative urine concentration of 11-dehydroTxB₂ was 106 ng/mmol of creatinine. On day 1, the concentration decreased only slightly and insignificantly (97 ng/ml, P = NS), similarly as on day 2 (86 ng/ml, P = NS). Only on day 5, significant decrease in concentration of thromboxane metabolite was achieved compared to preoperative values (46 ng/ml, P = 0.001). Conclusion Aspirin did not sufficiently inhibit platelet aggregation and thromboxane formation in the early postoperative period. Thus, antiplatelet treatment strategy should be intensified or modified in patients early after bypass surgery.