BACKGROUND In 2012, a total of 32 organizations entered the Pioneer accountable care organization (ACO) program, in which providers can share savings with Medicare if spending falls below a financial benchmark. Performance differences associated with characteristics of Pioneer ACOs have not been well described. METHODS In a difference-in-differences analysis of Medicare fee-for-service claims, we compared Medicare spending for beneficiaries attributed to Pioneer ACOs (ACO group) with other beneficiaries (control group) before (2009 through 2011) and after (2012) the start of Pioneer ACO contracts, with adjustment for geographic area and beneficiaries’ sociodemographic and clinical characteristics. We estimated differential changes in spending for several subgroups of ACOs: those with and those without clear financial integration between hospitals and physician groups, those with higher and those with lower baseline spending, and the 13 ACOs that withdrew from the Pioneer program after 2012 and the 19 that did not. RESULTS Adjusted Medicare spending and spending trends were similar in the ACO group and the control group during the precontract period. In 2012, the total adjusted per-beneficiary spending differentially changed in the ACO group as compared with the control group (−$29.2 per quarter, P = 0.007), consistent with a 1.2% savings. Savings were significantly greater for ACOs with baseline spending above the local average, as compared with those with baseline spending below the local average (P = 0.05 for interaction), and for those serving high-spending areas, as compared with those serving low-spending areas (P = 0.04). Savings were similar in ACOs with financial integration between hospitals and physician groups and those without, as well as in ACOs that withdrew from the program and those that did not. CONCLUSIONS Year 1 of the Pioneer ACO program was associated with modest reductions in Medicare spending. Savings were greater for ACOs with higher baseline spending than for those with lower baseline spending and were unrelated to withdrawal from the program. (Funded by the National Institute on Aging and others.)