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.