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Importance Medicare Advantage (MA) has entailed a major expansion of government-financed, privately administered health insurance in the US. As policy makers consider options to expand Medicare further, it is informative to compare the performance of traditional Medicare (TM) and MA. Objective To assess whether MA is associated with differential changes in health care utilization and spending for beneficiaries entering Medicare from commercial insurance compared with beneficiaries entering TM. Design, Setting, and Participants This retrospective cohort study with a difference-in-differences analysis and propensity score matching compared health care utilization and spending between beneficiaries enrolling in MA and beneficiaries enrolling in TM with a Medicare Supplement plan 1 year before vs 1 year after their initial Medicare enrollment. Participants included beneficiaries aged 65 to 70 years who remained enrolled with a large insurer when transitioning from commercial insurance to Medicare between June 2018 and December 2018. Data were analyzed from February 2020 to October 2021. Main Outcomes and Measures Use of, and spending on, institutional (Part A) and professional (Part B) medical services, measured as overall spending per member per month, and as rates of services per thousand members per year, including inpatient stays, inpatient days, physician visits, and injectable drug administrations. Results Among 1082 matched beneficiaries (541 joining MA, 541 joining TM with a Supplement plan), 585 (54.1%) were female, and the mean (SD) age at Medicare enrollment was 66 (1.4) years. Prior to Medicare enrollment, there was no statistically significant difference in outcome trends between the MA and TM groups. The first year of MA enrollment was associated with a differential reduction in institutional (Part A) spending of $95 (95% CI, $7-$183) per member per month, corresponding to a differential reduction in inpatient stays of 63 (95% CI, 10-116) per thousand members per year. Medicare Advantage was associated with a differential reduction in total spending (Parts A and B) of $142 (95% CI, $0-$282) per member per month, which was 36% of total spending in TM. There was no differential reduction in professional (Part B) spending (per member per month, $47; 95% CI, $51-$145) or utilization. Conclusions and Relevance In this cohort study with a difference-in-differences analysis, during the first year of Medicare coverage, MA was associated with large reductions in institutional (Part A) utilization and spending.