As part of the Ryan White HIV/AIDS program, the AIDS Drug Assistance Program (ADAP) serves as a payer of last resort for people living with HIV (PLWH) who are uninsured or underinsured, and have a low income. ADAP provides recipients with access to life-saving antiretroviral treatments which transforms an HIV diagnosis from a death sentence to a manageable chronic health condition. The ADAP program is funded through a combination of federal and state funds but is administered by the state. We exploit a rule change which was part of the 2006 reauthorization of the Ryan White Care Act and resulted in an exogenous shock to federal funding for ADAPs. Using this unique setting, the study finds that changes in federal contributions to ADAPs have a near dollar-to-dollar effect on expenditures, and increased federal funding results in an increased number of clients served. These findings underscore the importance of federal support for ADAPs in providing critical care for people living with HIV.