The Accountable Care Organization model has been touted as the innovative alternative to the current fee-for-service payment system. But as a new and transformative idea in health care, there hasn’t been much data on how it’s working. Now, health care policy researchers have released a national study of one hundred seventy- three ACOs that puts numbers to the doctor buy-in of the new model, and concludes that leadership from M.D.s may be the key to its success. Reporting in Health Affairs, researchers found that 51 percent of ACOs are physician-led, with another 33 percent jointly led by physicians and hospitals. In 78 percent of ACOs, physicians make up a majority of the governing board, and physicians own the equipment and employ the staff in 40 percent of the new organizations. Two-thirds of physician-led ACOs participate in the Medicare shared savings program.