The Centers for Medicare and Medicaid Services (CMS) is promoting formation of accountable care organizations (ACOs). In these population-based models, CMS aligns a Medicare beneficiary population to an ACO with associated expenditure and quality targets, transitioning away from purely volume-based revenue of fee-for-service Medicare. Patients with mental illness are among high-cost Medicare beneficiaries, but this population has received little attention in ACO implementation. Although the ACO goals of providing chronic and preventive care in a coordinated, patient-centered manner are consistent with what some mental health providers have long advocated, the population-based orientation may be unfamiliar. In addressing the needs of high-cost, high-risk patients to meet quality and expenditure targets, an ACO should examine the quality of mental health care it provides as well as medical quality for patients with mental illness. In addition, federal agencies should invest to ensure understanding of the impact of population-based initiatives on patients with mental illness.