This study will enroll people who have had a heart attack in the 2 to 4 weeks before study entry. At a baseline study visit, participants will undergo an MRI of their heart and complete an exercise stress test on a treadmill. A blood sample will be collected, and women will provide a urine sample for a pregnancy test. Questionnaires asking about diet, medical history, and emotions will also be completed. Participants will then be randomly assigned to receive either fish oil supplements or placebo on a daily basis for 6 months. Every 2 to 3 months, study researchers will call participants to check on their health and progress. At a study visit at Month 6, participants will undergo repeat baseline testing. After this visit, study researchers will call participants every 6 months for 3 years to follow up on participants' health status.