What Is an Accountable Care Organization, and Why Does It Matter for Your Primary Care?

By Marc Gilman

For additional information, please call: (800) 927-9326 or email:

Key Takeaways:

• An Accountable Care Organization (ACO) is a network of doctors, hospitals, and other providers who voluntarily coordinate your care and share responsibility for its quality and cost — it is not a health plan you enroll in, and it doesn’t change your benefits or cost you anything extra.

• If you have traditional Medicare, you may already be aligned with an ACO based on which primary care doctor you see most — often without realizing it.

• Massachusetts has 17 MassHealth ACOs operating under three different structural models, alongside separate Medicare ACOs — meaning “ACO” doesn’t mean just one thing in this state.

• The core financial idea behind every ACO model is the same: providers earn more by keeping you healthy and avoiding unnecessary care, not by billing for more procedures.

• Being in an ACO generally means better care coordination — particularly valuable if you see multiple specialists or manage several ongoing health conditions.

If you’ve heard the term “Accountable Care Organization” and weren’t quite sure what it meant — or whether it applies to you — you’re not alone. It’s one of the more consequential shifts in how primary care actually works, and it’s largely invisible to the people it affects most.

What Is an ACO, in Plain Terms?

An ACO is a group of doctors, hospitals, and other healthcare providers who agree to work together and take shared responsibility for the quality and cost of care for a defined group of patients. The idea is straightforward: instead of being paid simply for the volume of services delivered, providers in an ACO can earn additional payment when they deliver high-quality care at a lower total cost — which rewards prevention, coordination, and catching problems early, rather than more office visits, tests, or procedures.

Importantly, an ACO is not an insurance plan. You don’t enroll in one the way you’d enroll in a Medicare Advantage plan. There’s no separate premium, no new card, and typically no network restriction beyond the doctors you were already seeing. It’s a payment and coordination structure operating in the background of the care you’re already receiving.

Medicare’s ACOs: MSSP and ACO REACH

If you’re on traditional (original) Medicare — not a Medicare Advantage plan — you may already be aligned with an ACO without having done anything to sign up.

The largest program is the Medicare Shared Savings Program (MSSP), established under the Affordable Care Act. As of early 2025, roughly 476 ACOs were participating nationally, covering over 11 million traditional Medicare beneficiaries. A newer, higher-accountability model called ACO REACH (Realizing Equity, Access, and Community Health) is also expanding, with a particular focus on patients with complex, chronic conditions.

Here’s the part that surprises people: you’re generally assigned to an ACO based on which primary care provider you see most, determined by claims data — not by any application or election on your part. If you haven’t designated a “main doctor” with Medicare, that assignment happens automatically based on your care patterns. Medicare Advantage enrollees are not part of this system; MSSP and REACH apply specifically to traditional Medicare.

CMS has stated a goal of having every traditional Medicare beneficiary in an accountable care relationship by 2030 — so this is a trend that will only become more relevant, not less.

Massachusetts Has Its Own ACO System Too — With Three Different Models

Separately from Medicare, MassHealth (Massachusetts’ Medicaid program) launched its own ACO program in 2018. As of the most recent count, 17 MassHealth ACOs cover more than half of all MassHealth members. But unlike Medicare’s relatively uniform structure, MassHealth ACOs operate under three distinct models:

• Model A (Accountable Care Partnership Plans) — A provider organization partners with a single MassHealth managed care plan (MCO) to jointly form the ACO.

• Model B (Primary Care ACOs) — Providers contract directly with MassHealth, with no managed care plan involved as an intermediary. This model has an exclusive group of participating primary care providers.

• Model C (MCO-Administered ACOs) — A provider organization participates in the primary care network of one or more MassHealth managed care plans, which remain the member’s health plan.

Since April 2023, MassHealth has also required its ACOs to pay primary care practices through sub-capitation — a monthly per-member payment rather than fee-for-service billing — specifically to make sure the financial incentives that reward coordinated, preventive care actually reach the primary care level, not just the hospital system level.

If you’re a MassHealth member, this means your specific ACO’s structure can genuinely differ from a friend’s or neighbor’s ACO, even though both are called “ACOs.”

Why This Is Actually an Advantage for You

The financial mechanics behind an ACO can sound abstract, but the practical benefits tend to show up in fairly concrete ways:

• Better coordination between providers. ACOs are built around information-sharing between your primary care doctor, specialists, and hospitals — reducing duplicate tests and conflicting treatment plans.

• A stronger incentive toward prevention. Because providers benefit financially from keeping you healthy rather than treating avoidable complications, there’s a structural incentive toward proactive, preventive care.

• No cost or enrollment burden to you. Since it isn’t a plan, there’s no premium, no new ID card, and generally no new network restriction to navigate.

• Particularly valuable with multiple health conditions. If you’re managing several chronic conditions or see multiple specialists, the coordination an ACO is designed to encourage tends to matter more, not less.

What To Do Next

Whether you’re on traditional Medicare or MassHealth, it’s worth finding out whether your primary care provider participates in an ACO, and if so, which model or program it falls under — since that shapes how your care is coordinated behind the scenes.

If you need further information about how an ACO might apply to your specific Medicare or MassHealth coverage, we’re here to help — no cost, no obligation.

For additional information, please call: (800) 927-9326 or email: