Treatment of patients who have had a heart attack with drugs that prevent formation of blood clots has been shown to reduce the patient's risk of subsequent cardiovascular events such as heart attack, stroke, and death. Because new drugs have increased treatment options, the development of tests that can guide treatment should improve treatment selection and further reduce the risk of cardiovascular events as well as bleeding. This study is designed to assess the value of new tests. It is a prospective study that will enroll patients who have had a heart attack. Blood will be taken during hospitalization for a heart attack (1 day after their heart attack) and a second time 6 months later during an ambulatory clinical visit. Investigators will perform biochemical tests on the blood that assess the likelihood of making blood clots. One tablespoon of blood will be taken at each time. Taking this amount of blood poses no risk to the participant. Investigators will ask the participant whether they have had bleeding or cardiovascular events during the initial evaluation, the ambulatory follow-up at 6 months, and during a telephone interview 1 year after enrollment. During their 1 year of participation, investigators will review medical records and record information in a manner that protects the identity of all participants. We hypothesize that the biochemical test results will be similar at baseline and 6 month follow-up and that these biochemical tests will identify patients at greater risk of cardiovascular events and bleeding. Treatment of participants will not be altered by their participation in this study.