The purpose of this study is to determine the clinically meaningful definition and prevalence of aspirin resistance based on data linking aspirin-dependent laboratory tests to recurrent vascular events in cardiovascular patients taking aspirin.

Primary Outcomes: The primary endpoint is the composite outcomes of cerebral infarction, transient ischemic attack, myocardial infarction, unstable angina, percutaneous coronary angiography, thromboembolism, and death caused by cardiovascular disease.

Secondary Outcomes: Platelet aggregation induced by collagen and arachidonic acid, serum thromboxane B2, urine 11-dehydrothromboxane B2, platelet thrombus formation under flow conditions (sub-study only at the National Cardiovascular Center)

The primary objective of this study is to determine the markers for aspirin resistance. To achieve this objective, platelet aggregation, serum thromboxane B2, and urine 11-dehydrothromboxane B2 are measured in patients receiving usual therapeutic doses of aspirin, who experienced acute coronary syndrome, cerebral infarction, or transient ischemic attack occurred in a period between the past 1 month and the past 2 years. This is a multi-center, prospective study.

Inclusion Criteria:

Adult males or females who met the criteria listed below:

• Patients who had acute coronary syndrome, cerebral infarction (except cardioembolic stroke), or transient ischemic attack between past 28 days and the past 2 years
• patients who receive long-term aspirin therapy (at least 28 days)
• Patients who are >=20 years of age
• Patients willing and able to give written informed consent

Exclusion Criteria:

• Malignancy or suspected malignancy
• Patients who showed platelet counts less than 100,000/uL or more than 450,000/uL
• Congenital bleeding tendency
• Patients who receive other antiplatelet drugs or warfarin
• Patients with atrial fibrillation
• Patients who received surgical operation or catheter intervention within the past 2 weeks
• Patients who received anticoagulation drug therapy including heparin ( low-molecular weight heparin) and danaparoid.
• Patients with more than modified Rankin scale 4

out-patient clinic