Background: Trends in the use of guideline-based treatment for acute myocardial infarction (AMI) as well as its association with patient outcomes have not been summarized in a large, longitudinal study. Furthermore, it is unknown whether gender-, race-, and age-based care disparities have narrowed over time. Methods and Results: Using the National Registry of Myocardial Infarction database, we analyzed 2,515,106 patients with AMI admitted to 2,157 US hospitals between July 1990 and December 2006 to examine trends overall and in select subgroups of guideline-based admission, procedural, and discharge therapy use. The contribution of temporal improvements in acute care therapies to declines in in-hospital mortality was examined using logistic regression analysis. From 1990 to 2006, the use of all acute guideline-recommended therapies administered rose significantly for patients with ST-segment elevation myocardial infarction and patients with non-ST-segment myocardial infarction but remained below 90% for most therapies. Cardiac catheterization and percutaneous coronary intervention use increased in patients with ST-segment elevation myocardial infarction and patients with non-ST-segment myocardial infarction, whereas coronary bypass surgery use declined in both groups. Despite overall care improvements, women, blacks, and patients ≥75 years old were significantly less likely to receive revascularization or discharge lipid-lowering therapy relative to their counterparts. Temporal improvements in acute therapies may account for up to 37% of the annual decline in risk for in-hospital AMI mortality. Conclusion: Adherence to American Heart Association/American College of Cardiology practice guidelines has improved care of patients with AMI and is associated with significant reductions in in-hospital mortality rates. However, persistent gaps in overall care as well as care disparities remain and suggest the need for ongoing quality improvement efforts. © 2008 Mosby, Inc. All rights reserved.