More Medicare beneficiaries are in accountable care organizations than ever before. Participation is the metric that gets reported, but it is not the metric that matters most.
Each year brings a familiar headline: accountable care organizations cover more people than they did the year before. For 2026, CMS reports that roughly 14.3 million people in Traditional Medicare receive care through an ACO, up from 13.7 million in 2025. The number of Shared Savings Program ACOs has climbed to 511, the most in the program’s history, and a record share of them have taken on downside financial risk.
That growth is worth taking seriously. It is also worth being careful about, because participation is the easiest thing to count and the least revealing thing to celebrate.
What the growth actually signals
Some of what the numbers show is genuinely encouraging. When more than four in five Shared Savings Program ACOs sit in risk-bearing tracks, it means a large and growing set of providers has agreed to be held financially accountable when spending runs high and care falls short. That is a real commitment, not a press release. Accountable care has also proven durable across administrations and market cycles, which is not something every payment reform can claim.
Scale matters as well. A model covering more than 14 million people is no longer an experiment at the margin. It is a meaningful share of Traditional Medicare, with the reach to change how a large slice of American medicine is organized.
Why participation is not the same as impact
The trouble is that a headcount tells you who signed up, not what happened to them. In Performance Year 2024, Shared Savings Program ACOs earned $4.1 billion in shared savings and saved Medicare a net $2.5 billion. Against a program of this size, those are real but modest figures, and program-wide averages conceal enormous variation. Some ACOs generate substantial savings and better care. Others move very little. A few lose money. The average hides a distribution that is the actual story.
This is the same tension surfacing across value-based care right now. Adoption, investment, and enrollment are all climbing quickly. The evidence that they translate into better outcomes at lower cost is climbing more slowly, and less evenly. Counting inputs is easy. Measuring what they produce is hard.
Growth toward what?
None of this is an argument for less accountable care. It is an argument for judging it by the right yardstick. The question that matters is not how many people an ACO covers, but whether the people inside it are healthier, and whether the dollars spent on their behalf go further than they otherwise would have.
Those are harder things to know, and they depend on the unglamorous infrastructure that headline numbers skip past: reliable measurement, clean data, and the ability to tell a genuine improvement from a coding change or a favorable risk pool. Growth without that foundation can produce a program that is large and busy without being demonstrably better.
So the widening base is good news worth noting, and an incomplete story worth finishing. The next decade of accountable care will be judged less by how many join and more by what joining changes. That is the number worth watching, and the one that is hardest to move.