The aim of the present study was to assess the responsiveness of blood platelets to aspirin in patients following coronary artery bypass grafting (CABG) surgery. Aspirin was administered following CABG in 24 operated patients (aged 63.2 +/- 6.3 years). Platelet function was monitored on the 10th postoperative day (A) and 1 month after CABG (B) with the use of whole-blood aggregometry (WBEA) and PFA-100 closure time (PFA-100 CTCEPI). Normal platelet response to aspirin was defined by 3 criteria: the complete inhibition of WBEA induced by arachidonic acid (0.5 mmol/L), partial inhibition of collagen (1 microg/mL)-induced aggregation (WBEA < 14 Omega), and prolongation of PFA-100 CTCEPI (>150 seconds) ("good responders"). Depending on whether 0, 1, 2, or 3 of these 3 criteria were fulfilled, patients were classified as "nonresponders," "weak responders," "incomplete responders," or "good responders," respectively. On the 10th postoperative day, there were 3 good responders, 6 incomplete responders, 11 weak responders, and 4 nonresponders among 24 patients. In contrast, 1 month after CABG within the same group of 24 patients there were 18 good responders, 5 incomplete responders, and 1 weak responder. Using a new methodology to assess impaired platelet responsiveness to aspirin ex vivo, we describe here the transient nature of "aspirin resistance" following CABG. These results indicate the necessity for the prolonged monitoring of the antiplatelet effectiveness of aspirin in the postoperative period after CABG.