Background—Coronary heart disease (CHD) mortality rates have fallen dramatically over the past 4 decades in the Western world. However, recent data from the United States and elsewhere suggest a plateauing of CHD incidence and mortality among young women. We therefore examined recent trends in CHD mortality rates in the United States according to age and sex.Methods and Results—We analyzed mortality data between 1979 and 2011 for US adults ≥25 years of age. We calculated age-specific CHD mortality rates and compared estimated annual percentage changes during 3 approximate decades of data (1979–1989, 1990–1999, and 2000–2011). We then used Joinpoint regression modeling to assess changes in trends over time on the basis of inflection points of the mortality rates. Adults ≥65 years of age showed consistent mortality declines, which became even steeper after 2000 (women, −5.0%; men, −4.4%). In contrast, young men and women (<55 years of age) initially showed a clear decline in CHD mortality from 1979 until 1989 (estimated annual percentage change, −5.5% in men and −4.6% in women). However, the 2 subsequent decades saw stagnation with minimal improvement. Notably, young women demonstrated no improvements between 1990 and 1999 (estimated annual percentage change, 0.1%) and only −1% estimated annual percentage change since 2000. Joinpoint analyses provided consistent results.Conclusions—The dramatic decline in CHD mortality since 1979 conceals major heterogeneities. CHD death rates in older groups are now falling steeply. However, young adults have experienced frustratingly small decreases in CHD mortality rates since 1990. The drivers of these major differences in CHD mortality trends by age and sex merit urgent study.