ABSTRACT: Importance:Medicare beneficiaries who are also enrolled in Medicaid (dually enrolled beneficiaries) have drawn the attention of policy makers because they comprise the poorest subset of the Medicare population; however, it is unclear how their outcomes have changed over time compared with those only enrolled in Medicare (nondually enrolled beneficiaries). Objective:To evaluate annual changes in all-cause mortality, hospitalization rates, and hospitalization-related mortality among dually enrolled beneficiaries and nondually enrolled beneficiaries. Design, Setting, and Participants:Serial cross-sectional study of Medicare fee-for-service beneficiaries aged 65 years or older between January 2004 and December 2017. The final date of follow-up was September 30, 2018. Exposures:Dual vs nondual enrollment status. Main Outcomes and Measures:Annual all-cause mortality rates; all-cause hospitalization rates; and in-hospital, 30-day, 1-year hospitalization-related mortality rates. Results:There were 71?017?608 unique Medicare beneficiaries aged 65 years or older (mean age, 75.6 [SD, 9.2] years; 54.9% female) enrolled in Medicare for at least 1 month from 2004 through 2017. Of these beneficiaries, 11?697?900 (16.5%) were dually enrolled in Medicare and Medicaid for at least 1 month. After adjusting for age, sex, and race, annual all-cause mortality rates declined from 8.5% (95% CI, 8.45%-8.56%) in 2004 to 8.1% (95% CI, 8.05%-8.13%) in 2017 among dually enrolled beneficiaries and from 4.1% (95% CI, 4.08%-4.13%) in 2004 to 3.8% (95% CI, 3.76%-3.79%) in 2017 among nondually enrolled beneficiaries. The difference in annual all-cause mortality between dually and nondually enrolled beneficiaries increased between 2004 (adjusted odds ratio, 2.09 [95% CI, 2.08-2.10]) and 2017 (adjusted odds ratio, 2.22 [95% CI, 2.21-2.23]) (P?