A number of Medicare Shared Savings Program (MSSP) accountable care organizations have experienced significant, unanticipated changes in their historical benchmarks and performance expenditures for 2017. These changes were not consistent in direction or magnitude. We believe the exclusion of some nursing facility visits from MSSP assignment, effective in 2017, is the likely cause of the unanticipated changes. This article describes the assignment change and its possible impact.