Among Medicare beneficiaries, Affordable Care Act coverage expansions reduced utilization of ambulatory care, particularly among duals.

We leveraged local area variation in the size of the Affordable Care Act (ACA) expansions of Medicaid and nongroup coverage and measured changes in Medicare utilization and spending from 2010 through 2018 using the universe of Medicare fee-for-service claims. We found that the ACA coverage expansions led to decreases in the share of Medicare beneficiaries receiving ambulatory care and decreases in spending per beneficiary on ambulatory care. The reductions in ambulatory care were larger among beneficiaries enrolled in both Medicare and Medicaid ("duals"). Our results suggest that coverage expansions may lead to congestion and reduced access to physicians for those who are continuously insured.