Objective To determine how the inclusion of post‐acute evaluation and management (E&M) services as primary care affects assignment of Medicare beneficiaries to accountable care organizations (ACOs). Data Sources Medicare claims for a random 5 percent sample of 2009 Medicare beneficiaries linked to American Medical Association Group Practice data identifying provider groups sufficiently large to be eligible for ACO program participation. Study Design We calculated the fraction of community‐dwelling beneficiaries whose assignment shifted, as a consequence of including post‐acute E&M services, from the group providing their outpatient primary care to a different group providing their inpatient post‐acute care. Principal Findings Assignment shifts occurred for 27.6 percent of 25,992 community‐dwelling beneficiaries with at least one post‐acute skilled nursing facility stay, and they were more common for those incurring higher Medicare spending. Those whose assignment shifted constituted only 1.3 percent of all community‐dwelling beneficiaries cared for by large ACO‐eligible organizations (n = 535,138), but they accounted for 8.4 percent of total Medicare spending for this population. Conclusions Under current Medicare assignment rules, ACOs may not be accountable for an influential group of post‐acute patients, suggesting missed opportunities to improve care coordination and reduce inappropriate readmissions.