Active Medicare ACOs With the Earliest Recorded Agreement Start Dates in 2026
2026 Medicare Shared Savings Program organization snapshot · CMS file retrieved September 23, 2026
Three ACOs in the 2026 Medicare Shared Savings Program organization file share its earliest recorded Initial_Start_Date: Atlantic ACO, West Florida ACO, LLC, and Optimus Healthcare Partners, LLC, each dated April 1, 2012. This article ranks the date reported in the 2026 CMS file. It does not certify which organizations first entered Medicare's ACO program earliest across all prior identities or prove continuous participation. Source: CMS 2026 ACO organization file.
The earliest recorded dates in the 2026 roster
| Rank | ACO (2026 CMS name) | ACO ID | Recorded agreement start | Re-entering flag |
|---|---|---|---|---|
| 1 | Atlantic ACO | A1026 | 2012-04-01 | No |
| 1 | West Florida ACO, LLC | A1041 | 2012-04-01 | No |
| 1 | Optimus Healthcare Partners, LLC | A1059 | 2012-04-01 | No |
| 4 | PBACO Holding, LLC | A1001 | 2012-07-01 | No |
| 4 | Advocate Health Enterprise ACO | A1033 | 2012-07-01 | No |
| 4 | Ascension Care Management | A1061 | 2012-07-01 | No |
| 4 | Summa Accountable Care Organization | A1063 | 2012-07-01 | No |
| 4 | Arvon CIN | A1095 | 2012-07-01 | No |
| 4 | Florida Medical Clinic ACO, LLC | A1131 | 2012-07-01 | No |
| 4 | Memorial Hermann Accountable Care Organization | A1175 | 2012-07-01 | No |
| 4 | Methodist Alliance for Patients and Physicians | A1196 | 2012-07-01 | No |
| 4 | WESTMED Medical Group | A1199 | 2012-07-01 | No |
| 4 | BJC HealthCare ACO, LLC | A1204 | 2012-07-01 | No |
| 4 | Mercy Health Select, LLC | A1215 | 2012-07-01 | No |
| 4 | Cumberland Center for Healthcare Innovation | A1225 | 2012-07-01 | No |
| 4 | AnewCare Collaborative, LLC | A1241 | 2012-07-01 | No |
| 4 | Barnabas Health Care Network | A1247 | 2012-07-01 | No |
| 4 | University Hospitals Coordinated Care Organization | A1262 | 2012-07-01 | No |
| 4 | MaineHealth Accountable Care Organization | A1290 | 2012-07-01 | No |
| 4 | Asian American Accountable Care Organization, LLC | A1392 | 2012-07-01 | No |
| 4 | TP-ACO L.L.C. | A1401 | 2012-07-01 | No |
| 4 | AHN Accountable Care Organization, LLC | A1403 | 2012-07-01 | No |
| 23 | POM ACO | A1297 | 2013-01-01 | No |
| 23 | Lahey Clinical Performance ACO | A1490 | 2013-01-01 | No |
| 23 | Indiana Lakes ACO | A1494 | 2013-01-01 | No |
| 23 | AMITA Health Accountable Care Organization, LLC | A1501 | 2013-01-01 | No |
| 23 | Southcoast Accountable Care Organization, LLC | A1522 | 2013-01-01 | No |
| 23 | Ochsner Accountable Care Network, LLC | A1535 | 2013-01-01 | No |
| 23 | Orlando Health Collaborative Care | A1539 | 2013-01-01 | No |
| 23 | MCM ACCOUNTABLE CARE ORGANIZATION, LLC | A1557 | 2013-01-01 | No |
| 23 | Accountable Care Coalition of Georgia, LLC. | A1596 | 2013-01-01 | No |
| 23 | Billings Clinic | A1652 | 2013-01-01 | No |
| 23 | HHC ACO Inc | A1675 | 2013-01-01 | No |
| 23 | American Health Alliance, LLC | A1702 | 2013-01-01 | No |
| 23 | The Premier HealthCare Network LLC | A1737 | 2013-01-01 | No |
| 23 | HARTFORD HEALTHCARE ACCOUNTABLE CARE ORGANIZATION, INC. | A1749 | 2013-01-01 | No |
| 23 | ProHealth Physicians ACO, LLC | A2000 | 2013-01-01 | No |
This is the top-25-position list with all ties included. It contains 37 ACOs: three at April 1, 2012; 19 at July 1, 2012; and 15 tied at January 1, 2013. Because 15 organizations share rank 23, stopping at row 25 would arbitrarily remove 12 ACOs with the same date. Equal dates share a competition rank; tied rows appear in ACO ID order. None of these 37 rows has the 2026 re-entering flag set to yes. The full 511-ACO ranking is saved with the editorial evidence.
Why the title is precise
The 2026 organization file provides Initial_Start_Date and a Re-entering_ACO field. The CMS ACO data dictionary is the reference for these fields. An agreement start date in a current roster is useful for comparing recorded dates, but a re-entering organization's current record can fail to show its earliest appearance in the program. Renewal, gaps, and changes in identity also complicate any claim that an ACO has operated continuously since a displayed date.
We reviewed CMS annual organization files from 2014 through 2026, including both 2019 releases, using exact ACO IDs where present. The 2014 and 2015 files in the accessible series did not supply the same ACO ID field; 2016 onward provided usable IDs. We did not join older records by similar names because names can change or collide. Among 2026 roster records marked re-entering, 98 had no earlier, same-ID date recovered by this audit. Without an established identity match for every possible early entrant, a headline asserting the definitive “oldest Medicare ACOs” would be stronger than the evidence. The 2026-field ranking is the brief's permitted fallback.
Historical files do provide context for the displayed group: each of the 37 had earlier annual rows under the same ACO ID in the files we inspected, and their earliest observed date agreed with the date shown in the 2026 file. That check supports the recorded date for these IDs. It does not resolve whether a different current ID represents an organization that entered before its present agreement or whether participation was uninterrupted. The CMS program data page is the starting point for the historical releases and crosswalk needed for a stronger lineage analysis.
What a start date can tell you
A date is an identifier of the agreement record's chronology, not a quality score. The ACOs at the top of this list need not have the largest participant rosters or shared savings payments. For example, Optimus Healthcare Partners shares the earliest date yet is rank 14 by distinct participant-name count in our separate 2026 roster analysis. That comparison is within the same program but uses a different measurement. Avoid treating longevity as evidence of patient reach, financial success, or organizational continuity.
Names are also a weak historical key. A1001 is called “PBACO Holding, LLC” in 2026, while a 2024 annual file under the same ACO ID uses “Palm Beach Accountable Care Organization.” A change in label is a reason to retain the stable ID and year in a research worksheet. It is not, by itself, evidence of a merger, ownership change, or uninterrupted membership. Provider and participant rosters can change even when an ACO ID persists.
Methodology
We treated every unique ACO_ID in the 2026 CMS organization file as a rostered ACO. All 511 had a nonblank Initial_Start_Date. We parsed those dates and sorted ascending, assigning the same competition rank to equal dates. The re-entry flag is shown so the reader can separate a record's date from an assumption about first-ever entry. The 2026 CSV is in CMS's April 2026 release path and was retrieved September 23, 2026; the roster is a dated snapshot, not a live status service. We also saved a historical same-ID audit, coverage and unresolved-ID summary, and source manifest with the editorial evidence. The audit did not resolve pre-ID records or all re-entry histories, so the table deliberately uses only the 2026 field. A full “oldest since first participation” article would require the official crosswalk, resolution of ambiguous identities, and evidence for any gaps or re-entry across the complete historical series.
Explore ACO Networks in QOPE
For a specific ACO, explore its recorded start year and network details in QOPE, then check the linked CMS source year before making a historical claim.