OBJECTIVES:To determine the willingness of accountable care organizations (ACOs) to bear financial risk for the healthcare they provide. DESIGN AND METHODS:Structured interviews conducted between January and June 2012 with 57 ACOs led by hospitals and physician groups located throughout the United States. Findings are based on the 38 ACOs that were actively providing care under an ACO payment arrangement at the time of the interview. RESULTS:Among these ACOs, 71% cover a portion of their ACO population with contracts that put the ACOs at some financial risk, while 45% have risk-based contracts for their entire ACO population. Payments based on fee-for-service (FFS) billing still dominate, as 92% of ACOs use FFS-based billing for at least a portion of their ACO population and 71% are fully reimbursed using FFS-based billing. CONCLUSIONS:Under the auspices of an ACO, providers are accepting some financial risk for their accountable care patient population. There is still strong reliance on FFS-based billing methods as providers experiment with different payment models.