Objective: To examine changes in more and less discretionary condition-specific post-acute care use (skilled nursing, inpatient rehabilitation, home health) associated with Medicare accountable care organization (ACO) implementation. Data sources: 2009-2014 Medicare fee-for-service claims. Study Design: Difference-indifference methodology comparing post-acute outcomes after hospitalization for hip fracture and stroke (where rehabilitation is fundamental to the episode of care) to pneumonia, (where it is more discretionary) for beneficiaries attributed to ACO and non-ACO providers. Principal Findings: Across all three cohorts, in the baseline period ACO patients were more likely to receive Medicare-paid post-acute care and had higher episode spending. In hip fracture patients where rehabilitation is standard of care, ACO implementation was associated with 6-8% increases in probability of admission to a skilled nursing facility or inpatient rehabilitation (compared to home without care), and a slight reduction in readmissions. In a clinical condition where rehabilitation is more discretionary, pneumonia, ACO implementation was not associated with changes in post-acute location, but episodic spending decreased 2-3%. Spending decreases were concentrated in the least complex patients. Across all cohorts, the length of stay in skilled nursing facilities decreased with ACO implementation. Conclusions: ACOs decreased spending on post-acute care by decreasing use of discretionary services. ACO implementation was associated with reduced length of stay in skilled nursing facilities, while hip fracture patients used institutional post-acute settings at higher rates. Among pneumonia patients, we observed decreases in spending, readmission days, and mortality associated with ACO implementation.