Accountable Care Organizations (ACOs) are the most-widespread of the Centers for Medicare & Medicaid Services (CMS) value-based payment programs and demonstrations. Several ACO demonstrations have been released by CMS's Innovation Center, many modifying previous demonstrations, or building on demonstrations to be more inclusive of small and rural providers. As a result, ACOs have expanded broadly into rural areas, albeit with lower beneficiary attribution numbers than in urban areas.

Using CMS data on Medicare beneficiaries attributed to Shared Savings ACOs in US counties for 2014-2016 we find that nearly every county (or equivalent) in the US had at least one resident Medicare beneficiary attributed to an ACO in 2016. Over this period, there has been significant growth in ACO attribution in both metropolitan and non-metropolitan counties but the rate of growth in non-metropolitan counties has been substantially higher that in metropolitan counties.