The Centers for Medicare and Medicaid Services has implemented an inpatient rehabilitation facility prospective payment system (IRF-PPS) based on case-mix groups (CMGs). The CMGs, now almost identical in structure to the Functional Independence Measure-Function-Related Groups (FIM-FRGs), will measure patients' functional severity by the FIM trade mark instrument, rather than by the Minimum Data Set for Post-Acute Care, as was initially planned. Although this late change in plans is a major triumph for physical medicine and rehabilitation and for the patients we serve, economic incentives inherent in the IRF-PPS may still transform inpatient rehabilitation as it is currently practiced in the United States. This commentary compares the CMGs with the FIM-FRGs; addresses the implications of the CMGs' implementation for patients, researchers, and clinicians; and highlights ways of adapting previous FIM-FRG applications and research to help meet the challenges presented by the new IRF-PPS.