The Affordable Care Act made admirable strides toward the “triple aim” of reducing health care costs, increasing health care quality, and improving the health of the community at large. A key element of reform is the accountable care organization (ACO), which restructures health care delivery such that networks of providers are held responsible for a group of patients they serve. The recently announced Medicare ACO program lays the foundation for 2 of its 3 major goals by allowing ACOs to share in any cost savings, provided they meet quality criteria. Yet it seems that the public health goals of accountable care—arguably the most important of the 3—have been left behind. To better address public health goals, we propose a novel method for quality reporting within ACOs: introducing an “expanded denominator” that attributes patients to a health system if they have ever been seen within the system. An expanded denominator would ensure that ACOs are held accountable not only for patients already engaged in primary care but also for patients with fragmented care and high-risk community members not receiving adequate care. Ultimately, payment reform in Medicare, and potentially Medicaid, must support this new approach to quality measurement for it to have lasting ramifications.