Background: Previous studies have shown that women younger than 55 years have higher hospital mortality rates after acute myocardial infarction (MI) than agematched men. We examined whether such mortality differences have decreased in recent years. Methods: We investigated temporal trends in the hospital case-fatality rates of MI by sex and age from June 1, 1994, through December 31, 2006. The study population included 916 380 patients from the National Registry of Myocardial Infarction with a confirmed diagnosis of MI. Results: In-hospital mortality decreased markedly between 1994 and 2006 in all patients but more so in women than men. The mortality reduction in 2006 relative to 1994 was largest in women younger than 55 years (52.9%) and lowest in men younger than 55 years (33.3%). In patients younger than 55 years, the absolute decrease in mortality was 3 times larger in women than men (2.7% vs 0.9%). As a result, the excess mortality in youngerwomen(<55 years) compared withmenwas less pronounced in 2004-2006 (unadjusted odds ratio, 1.32; 95% confidence interval, 1.07-1.67) than it was in 1994-1995 (unadjusted odds ratio, 1.93; 95% confidence interval, 1.67-2.24). The sex difference in mortality decrease was lower in older patients (P=.004 for the interaction among sex, age, and year). Changes in comorbidity and clinical severity features at admission accounted for more than 90% of these mortality trends. Conclusions: In recent years, women, particularly younger ones, experienced larger improvements in hospital mortality after MI than men. The narrowing of the mortality gap between younger women and men is largely attributable to temporal changes in risk profiles. ©2009 American Medical Association. All rights reserved.