To define the risk factors and clinical presentation of patients under age 40 who present to the emergency department (ED) of a community hospital with an acute myocardial infarction (MI), a retrospective cross-sectional study was conducted over a 7-year period. Two hundred and nine consecutive cases of initial MI who met World Health Organization criteria (chest pain, ECG changes, and serum enzyme rises) and were admitted to one of five participating hospitals were reviewed. The mean age of patients was 34.8 years (range, 17-39); 81% were male. The major risk factor was tobacco use (81%), followed by family history (40%), hypertension (26%), and hyperlipidemia (20%). One hundred and eighty-three patients (87.6%) had ECG evidence of cardiac ischemia, injury, or infarction in the ED. Approximately 24% of patients had multi-vessel coronary atherosclerosis as documented by angiography; 62% had single vessel disease; and 14% had normal coronary arteries. The most common anatomical location for the MI was the inferior wall. This study characterized the epidemiology of acute MI in young adults: 1) smoking emerged as the main coronary risk factor; 2) atherosclerosis continues to be the major etiology; 3) a common finding on angiography was single-vessel disease causing infarction of the inferior wall; and 4) the complication rate was comparable to older populations, but the in-hospital mortality was only 1.9%.