Medicare Shared Savings Program (MSSP) accountable care organizations remain the most visible federal value-based care vehicle for Medicare fee-for-service beneficiaries. This analysis ranks 476 distinct MSSP parent organizations by U.S. headquarters state, layers participant rosters and county-level beneficiary assignment, and contrasts the separate ACO REACH program—giving payers, provider strategists, researchers, and life-sciences market access teams a single, decision-grade view of where Medicare ACO capacity sits today.
Executive briefing
- Universe: 476 MSSP organization IDs from CMS PY2025 organization file — not commercial or Medicaid ACO products.
- Geography: Headquarters state (street address) drives state rankings; multi-state service areas appear in county assignment and Explore data.
- Network scale: 15,192 participant organizations; ~31.9 per parent ACO.
- Attribution depth: 135,203 county-beneficiary rows; 476 MSSP ACOs assign beneficiaries across more than one state.
- REACH: 132 entities under ACO REACH (2023) — benchmark separately from MSSP.
- Explore: Signed-in readers can drill into directories and filters via Explore data.
MSSP accountable care organizations by state
Table 1 counts distinct MSSP parent organization IDs where CMS publishes a U.S. street address resolving to each state. Service-area states are not double-counted here; use county assignment (below) for dispersion.
| Rank | State | MSSP ACOs | Share of U.S. |
|---|---|---|---|
| 1 | MD | 59 | 12.4% |
| 2 | TN | 49 | 10.3% |
| 3 | FL | 41 | 8.6% |
| 4 | VA | 23 | 4.8% |
| 5 | NY | 22 | 4.6% |
| 6 | TX | 22 | 4.6% |
| 7 | CA | 20 | 4.2% |
| 8 | PA | 19 | 4.0% |
| 9 | IL | 15 | 3.2% |
| 10 | NJ | 15 | 3.2% |
| 11 | MA | 13 | 2.7% |
| 12 | MI | 12 | 2.5% |
| 13 | MO | 11 | 2.3% |
| 14 | OH | 11 | 2.3% |
| 15 | WA | 11 | 2.3% |
| 16 | NC | 10 | 2.1% |
| 17 | LA | 9 | 1.9% |
| 18 | WI | 9 | 1.9% |
| 19 | AZ | 8 | 1.7% |
| 20 | GA | 8 | 1.7% |
| 21 | IN | 8 | 1.7% |
| 22 | CT | 6 | 1.3% |
| 23 | CO | 5 | 1.1% |
| 24 | IA | 5 | 1.1% |
| 25 | MN | 5 | 1.1% |
| 26 | NE | 5 | 1.1% |
| 27 | SC | 5 | 1.1% |
| 28 | DE | 4 | 0.8% |
| 29 | MS | 4 | 0.8% |
| 30 | NH | 4 | 0.8% |
| 31 | OK | 4 | 0.8% |
| 32 | KS | 3 | 0.6% |
| 33 | KY | 3 | 0.6% |
| 34 | ME | 3 | 0.6% |
| 35 | MT | 3 | 0.6% |
| 36 | NV | 3 | 0.6% |
| 37 | RI | 3 | 0.6% |
| 38 | SD | 3 | 0.6% |
| 39 | AL | 2 | 0.4% |
| 40 | AR | 2 | 0.4% |
| 41 | ID | 2 | 0.4% |
| 42 | OR | 2 | 0.4% |
| 43 | WV | 2 | 0.4% |
| 44 | AK | 1 | 0.2% |
| 45 | HI | 1 | 0.2% |
| 46 | UT | 1 | 0.2% |
Table 1. MSSP organizations by headquarters state (PY2025). Source: CMS organization file.
Figure 1. Top states by MSSP organization count (headquarters address, PY2025)
MD accounts for 59 MSSP organizations (12.4% of the U.S. census), the largest headquarters concentration in PY2025. Payers and employers benchmarking ACO density should treat this as a registration geography signal—not beneficiary attribution, which Table 3 addresses.
MSSP participant organizations by state
Participant rosters list hospitals, physician groups, and other entities tied to parent ACO IDs. State counts below use the parent ACO’s published address—useful for estimating where legal entities concentrate even when beneficiaries are attributed elsewhere.
| Rank | State | Participants | Distinct parent ACOs |
|---|---|---|---|
| 1 | MD | 2,121 | 59 |
| 2 | FL | 1,752 | 41 |
| 3 | TN | 1,135 | 49 |
| 4 | NY | 1,004 | 22 |
| 5 | TX | 978 | 22 |
| 6 | MI | 773 | 12 |
| 7 | MA | 735 | 13 |
| 8 | CA | 642 | 20 |
| 9 | GA | 576 | 8 |
| 10 | IL | 487 | 15 |
| 11 | MO | 483 | 11 |
| 12 | PA | 478 | 19 |
| 13 | VA | 438 | 23 |
| 14 | OH | 406 | 11 |
| 15 | NJ | 399 | 15 |
Table 2. MSSP participant organizations by parent ACO headquarters state (PY2025). Source: CMS participant file.
Figure 2. MSSP participant organizations by ACO headquarters state (PY2025)
County beneficiary assignment and geographic span
The CMS assigned-beneficiaries-by-county public use file shows where Medicare FFS beneficiaries are attributed to each MSSP ACO. Wide county span with a narrow headquarters footprint often signals referral-leakage risk for narrow networks and medical-group strategists.
| Rank | ACO ID | Counties | Assigned beneficiaries | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| No data available. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Table 3. MSSP ACOs with the widest county footprints (latest county PUF vintage).
Across the latest county file, 476 MSSP ACOs have at least one county row (135,203 total rows). 476 parent ACOs assign beneficiaries in more than one state— material for regional payer overlap and multi-market provider systems.
MSSP vs ACO REACH: keep programs separate
Commercial slide decks often report “1,500+ ACOs” by blending programs. For Medicare-only strategy, MSSP and ACO REACH must be reported on their own terms.
| Program / layer | Count | Notes |
|---|---|---|
| Medicare Shared Savings Program (MSSP) | 476 | Parent ACO organizations (PY2025) |
| MSSP participant organizations | 15,192 | Hospitals, groups, and entities on participant roster |
| ACO REACH entities | 132 | Separate Innovation Center model (2023) — do not add to MSSP total |
| County assignment rows | 135,203 | Beneficiary-by-county PUF across 476 MSSP ACOs |
Table 4. Program scale comparison (latest ingested CMS vintages).
Among REACH entities in 2023, 108 are classified global-risk and 24 professional-risk — a different contracting and quality profile than MSSP tracks.
What this means for your segment
Payers and health plans
Use Table 1 for market density and Table 3 for attribution overlap when designing narrow networks or evaluating MSSP partners as downstream cost benchmarks. Do not infer beneficiary share from headquarters counts alone.
Provider systems and physician groups
Participant density (Table 2) highlights states where subsidiary entities cluster. Compare your service-area footprint to county assignment rows before assuming an MSSP parent in your state controls local attribution.
Researchers and policy analysts
Headquarters rankings answer “where organizations register”; county PUF answers “where beneficiaries are assigned.” Publish both lenses when modeling Medicare VBC penetration.
Biopharma, medtech, and market access
ACO lists are inputs to territory planning, not prescription volumes. Pair this geography with your own claims or formulary data; MSSP participation does not imply therapeutic class concentration.
Methodology and limitations
- MSSP organizations: CMS Data.CMS.gov organization CSV (PY2025); distinct
aco_id; state from street address. - Participants: CMS participant file linked to parent
aco_id. - County beneficiaries: CMS Data API / PUF for assigned beneficiaries by county (Explore for full export).
- REACH: CMS Innovation Center financial and quality PUF (2023).
- Not included: Commercial ACOs, Medicare Advantage plan networks, or Next Generation ACO legacy cohorts outside current CMS files.