Although acute myocardial infarction (AMI) occurs primarily in the elderly, this disease also affects young adults. Few studies have, however, presented data on relatively young patients hospitalized with AMI. The objectives of this population-based study were to examine recent trends in the magnitude, clinical characteristics, management, and in-hospital and long-term outcomes associated with ST-segment elevation myocardial infarction (STEMI) and non-ST-segment elevation myocardial infarction (NSTEMI) in patients aged 30 to 54 years. We reviewed the medical records of 955 residents of the Worcester (Massachusetts) metropolitan area aged 30 to 54 years who were hospitalized for an initial STEMI or NSTEMI in 6 biennial periods from 1999 to 2009 at 11 greater Worcester medical centers. From 1999 to 2009, the proportion of young adults hospitalized with an STEMI decreased from approximately 2/3 to 2/5 of all patients with an initial AMI. Patients with STEMI were less likely to have a history of heart failure, hypertension, hyperlipidemia, and kidney disease than those with NSTEMI. Both groups received similar effective medical therapies during their acute hospitalization. In-hospital clinical complications and mortality were low and no significant differences in these end points were observed between patients with STEMI and NSTEMI or with regard to 1-year postdischarge death rates (1.9% vs 2.8%). The present results demonstrate recent decreases in the proportion of relatively young patients diagnosed with an initial STEMI. Patients with STEMI and NSTEMI had similar in-hospital outcomes and long-term survival. Trends in these and other important outcomes warrant continued monitoring.