Background: Practice guidelines for acute ST-segment elevation myocardial infarction (STEMI) and non-STEMI (NSTEMI) recommend similar therapies and interventions, but differences in patterns of care between MI categories have not been well described in contemporary practice. Methods: In-hospital treatments with similar recommendations from practice guidelines were compared with outcomes in 185 968 eligible patients (without listed contraindications) with STEMI (n=53 417; 29%) vs NSTEMI (n=132 551; 71%) from 1247 US hospitals participating in the National Registry of Myocardial Infarction 4 between July 1, 2000, and June 30, 2002. Hierarchical logistic regression modeling was used to determine adjusted differences in treatment patterns in MI categories. Results: Unadjusted in-hospital mortality rates were high for NSTEMI (12.5%) and STEMI (14.3%), and the use of guideline-recommended medications and interventions was suboptimal in both categories of patients with MI. The adjusted likelihood of receiving early (within 24 hours of presentation) aspirin, β-blockers, and angiotensin-converting enzyme inhibitors was higher in patients with STEMI. Similar patterns of care were noted at hospital discharge: the adjusted likelihood of receiving aspirin, β-blockers, angiotensin-converting enzyme inhibitors, lipid-lowering agents, smoking cessation counseling, and cardiac rehabilitation referral was higher in patients with STEMI. Conclusions: Evidence-based medications and lifestyle modification interventions were used less frequently in patients with NSTEMI. Quality improvement interventions designed to narrow the gaps in care between NSTEMI and STEMI and to improve adherence to guidelines for both categories of patients with MI may reduce the high mortality rates associated with acute MI in contemporary practice. ©2005 American Medical Association. All rights reserved.