This report presents early findings from an evaluation of the 32 Pioneer Accountable Care Organizations’ (ACO) effects on Medicare spending between 2011 and 2012, the first year of the demonstration program. As part of a multi-method data collection and analytic approach, the evaluation estimates the ACO treatment effect using an approach akin to a difference-in-differences framework to compare the growth rate in per beneficiary per month Medicare spending for Pioneer ACO-aligned beneficiaries to two comparison groups: 1) fee-for-service (FFS) Medicare beneficiaries who are not aligned or assigned to a Medicare ACO in the Pioneer ACO’s local market and, 2) FFS Medicare beneficiaries in a geographically distinct, but similar market where Medicare ACOs are not present. Analyses were conducted primarily at the individual Pioneer ACO level, but we also report results averaged over all Pioneer ACOs, with each Pioneer ACO weighted by the number of aligned beneficiaries.