Objectives: Changes in population demographics and comorbid illness prevalence, improvements in medical care, and shifts in care delivery may be driving changes in the composition of patients admitted to the ICU. We sought to describe the changing demographics, diagnoses, and outcomes of patients admitted to critical care units in the U.S. hospitals. Design: Retrospective cohort study. Setting: U.S. hospitals. Patients: There were 27.8 million elderly (age, > 64 yr) fee-for-service Medicare beneficiaries hospitalized with an intensive care or coronary care room and board charge from 1996 to 2010. Interventions: None. Measurements and Main Results: We aggregated primary International Classification of Diseases, 9th Revision, Clinical Modification discharge diagnosis codes into diagnoses and disease categories. We examined trends in demographics, primary diagnosis, and outcomes among patients with critical care stays. Between 1996 and 2010, we found significant declines in patients with a primary diagnosis of cardiovascular disease, including coronary artery disease (26.6 to 12.6% of admissions) and congestive heart failure (8.5 to 5.4% of admissions). Patients with infectious diseases increased from 8.8% to 17.2% of admissions, and explicitly labeled sepsis moved from the 11th-ranked diagnosis in 1996 to the top-ranked primary discharge diagnosis in 2010. Crude in-hospital mortality rose (11.3 to 12.0%), whereas discharge destinations among survivors shifted, with an increase in discharges to hospice and postacute care facilities. Conclusions: Primary diagnoses of patients admitted to critical care units have substantially changed over 15 years. Funding agencies, physician accreditation groups, and quality improvement initiatives should ensure that their efforts account for the shifting epidemiology of critical illness.