BACKGROUND: Declining mortality rates of coronary heart disease in the United States could be attributable to declining incidence, declining severity, and/or improvements in treatment. METHODS: We examined trends in severity of patients hospitalized for myocardial infarction to characterize its contribution to this decline by using data from the Atherosclerosis Risk in Communities (ARIC) study. RESULTS: No significant change in the proportion having systolic blood pressure <100 mm Hg or an abnormal pulse at presentation was noted. The proportion with ST-segment elevation on the initial electrocardiogram increased 10% per year (P <.001), and the proportion with a diagnostic or evolving diagnostic electrocardiogram abnormality increased 4% per year (P <.01); the proportion that had a new Q-wave infarction develop remained unchanged. The mean peak creatine kinase level decreased 5% per year (P <.001), the proportion with abnormal enzyme levels decreased 10% per year (P <.001), and the proportion that met criteria for definite myocardial infarction decreased 4% per year (P <.05). The proportion that had cardiogenic shock decreased 10.9% per year (P <. 01), but the proportion that had an acute episode of congestive heart failure was stable. CONCLUSIONS: With stable hemodynamic indicators, worsening electrocardiographic indicators, and improving enzymatic indicators, these results provide mixed support for decreases in the severity of myocardial infarction.