Methods Selection of patients This prospective observational study will be conducted at the Institute of Cardiology in the Pitié-Salpêtrière Hospital, Paris, France. It is planned to start in december 2009 to november 2010. Care of patients will conform to standard procedures currently used in our institute, which will not be modified by the study. Platelet function will be measured in residual blood samples, after ethical committee approval (Comité de Protection des Personnes VI - Ile de France). A written informed consent is not resquested from patients. Nevertheless, written information will be delivered for each of them. During the planned 11 months of investigation, all patients undergoing isolated first-time coronary artery by-pass grafting will be enrolled. Patients admitted for emergency procedure, following failed percutaneous transluminal coronary angioplasty and off-pump coronary surgery, will be excluded. Because antithrombotic therapy management may differ during the postoperative period, we will exclude patients in whom a mechanical support or intra-aortic balloon pump will be required for a difficult weaning from cardiopulmonary bypass. Finally, we will exclude all patients who will not have received any oral antiplatelet agents within 5 days prior surgery, those who will have been preoperatively exposed to platelet glycoprotein IIb/IIIa inhibitors, and those with history of haematological disease. Patients treated with aspirin and clopidogrel and in whom clopidogrel will be stopped more than 5 days before surgery will also be excluded.