Myocardial infarction in persons under the age of 45 years accounts for 6% to 10% of all myocardial infarctions in the United States. In this age group, it is predominantly a disease of men. Important risk factors include a family history of myocardial infarction before age 55 years, hyperlipidemia, smoking, and obesity. Unlike older patients, approximately half of young patients have single-vessel coronary disease, and in up to 20%, the cause is not related to atherosclerosis. Coronary angiography may be warranted in young patients with myocardial infarction to define the anatomy of the disease and to permit optimal management.