The Functional Independence Measure-Function Related Groups (FIM-FRGs) classify stroke patients into five clinical categories that predict medical rehabilitation length of stay as a proxy for cost. The current payment system for inpatient rehabilitation does not adjust for clinical complexity, thus potentially leading to care inequities for individual patients and unequal distribution of resources among rehabilitation facilities. Designed for use as part of a payment system, the FIM-FRGs could operate within the Tax Equity and Fiscal Responsibility Act payment system or other payment systems to adjust for the clinical complexity of the patients actually treated. The article discusses the use of function-based classification schemes for the payment of stroke rehabilitation.