Medicare's Shared Savings Program (MSSP) had 511 accountable care organizations (ACOs) in 2026, up from 476 in 2025, according to the Centers for Medicare & Medicaid Services (CMS). Those 511 ACOs cover more than 700,000 health care providers and 12.6 million people with Traditional Medicare.
CMS approved 134 applications for the 2026 program year, 72 new ACOs and 62 renewing or reentering ones. Beneficiary enrollment in the Shared Savings Program rose 12.3% from 2025, and 82.8% of participating ACOs now sit in an advanced, two-sided risk track rather than a no-risk track. For a practice weighing whether to join an ACO, or whether to stay in one, these numbers describe a program that is still growing and still pushing participants toward more financial risk.
Shared Savings Reached $4.1 Billion for 2024
For performance year 2024, the most recently reconciled year, Shared Savings Program ACOs earned $4.1 billion in shared savings and saved the Medicare program $2.5 billion, CMS reported. Those totals were earned by the 476 ACOs that operated during the 2025 program year, before the count rose to 511 for 2026.
Across every CMS accountable care model combined, not just the Shared Savings Program, 14.3 million Medicare beneficiaries received ACO-coordinated care in 2026, up from 13.7 million in 2025, a 4.4% increase in a single year. CMS does not publish a per-ACO or per-practice average for the $4.1 billion figure, so a practice cannot divide that total by 511 and expect a realistic estimate of its own result.
Three Smaller Models Run Alongside the Shared Savings Program
CMS also runs three other accountable care models that a smaller number of practices use. ACO REACH had 74 participating ACOs in 2026, covering 125,909 providers and organizations and 1.7 million Traditional Medicare beneficiaries, including 614 federally qualified health centers, rural health clinics, and critical access hospitals. The Kidney Care Choices model had 74 Kidney Contracting Entities, 7,534 participating providers, and 237,000 beneficiaries with chronic kidney disease or end-stage renal disease.
The newest model, ACO Primary Care Flex, had 23 ACOs serving 359,720 beneficiaries in 2026. CMS pays each Primary Care Flex participant a one-time $250,000 advance, then replaces regular primary care fee-for-service payments with monthly prospective primary care payments set from a county-level rate book built on three years of historical spending. That shift changes what a billing team tracks day to day, since primary care visits inside the model stop generating a fee-for-service claim and instead roll up into the monthly payment.
| Model | ACOs | Providers/organizations | Beneficiaries |
|---|---|---|---|
| Shared Savings Program | 511 | 700,000+ | 12.6 million |
| ACO REACH | 74 | 125,909 | 1.7 million |
| Kidney Care Choices | 74 entities | 7,534 | 237,000 |
| ACO Primary Care Flex | 23 | Not separately reported | 359,720 |
Source: CMS, "2026 Medicare Accountable Care Organization Initiatives Participation Highlights," February 4, 2026.
The Path Into Risk Gets Shorter Starting in 2027
CMS finalized a change to the Shared Savings Program's BASIC track glide path in the CY2026 Medicare Physician Fee Schedule final rule. For agreement periods starting on or after January 1, 2027, a new ACO's maximum time in a one-sided, no-risk model within the BASIC track falls from 7 performance years to 5. CMS said the change is meant to move more ACOs into two-sided risk sooner.
The same rule sets a new floor on ACO size. ACOs must have at least 5,000 assigned beneficiaries in the third benchmark year of an agreement period, and any ACO that falls below 5,000 beneficiaries in a given year is limited to BASIC track participation, with shared savings and shared losses capped at lower amounts. For 2026, CMS also raised the Medicare Economic Index, the measure of practice cost inflation used to update physician pay, by 2.7%, a figure that feeds directly into how ACO benchmarks and fee schedule rates get set each year.
What the Numbers Mean for a Practice Deciding Whether to Join
None of these figures say whether a specific independent practice should join an ACO. They do show a program CMS is actively expanding and reshaping. The Shared Savings Program added 35 net ACOs and grew its provider count past 700,000 in 2026, while shortening the runway new entrants get before taking on downside risk. With 82.8% of current ACOs already in a two-sided track, and the one-sided period now capped at 5 years instead of 7, a practice joining today will spend less time than earlier cohorts operating without exposure to shared losses.
The 5,000-beneficiary minimum matters most for smaller independent groups. A single-specialty or small multispecialty practice sizing up ACO participation now has to weigh whether its own attributed panel, or a panel built jointly with other practices, can clear that floor by the third benchmark year, since falling short locks an ACO into the lower-risk, lower-reward BASIC track for the life of the agreement.
For billing and finance staff, the practical change is what gets measured. Fee-for-service claims volume and denial rates stay relevant for any care delivered outside a model's scope, but ACO participation adds attributed-panel size, quality measure reporting, and, inside ACO Primary Care Flex, a monthly prospective payment in place of some fee-for-service billing. Practices already reporting MIPS quality measures or managing prior authorization volume have a head start on the reporting infrastructure these models expect.
Sources
- CMS, "2026 Medicare Accountable Care Organization Initiatives Participation Highlights," February 4, 2026
- CMS, "Calendar Year (CY) 2026 Medicare Physician Fee Schedule Final Rule (CMS-1832-F), Medicare Shared Savings Program Changes"
- CMS, "ACO Primary Care Flex (ACO PC Flex) Model Frequently Asked Questions, Version 3"
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