This paper explores organizational dynamics that go with the design and implementation of public administration reforms within the UN system. It focuses on management reforms carried out in the UNAIDS Programme, which brings together ten UN agencies to combat the worldwide HIV/AIDS epidemic. The paper suggests that understanding these reforms requires questioning the exposure of UN administrations to pressures emanating from their environment and, at the same time, investigating the intentions of bureaucratic entrepreneurs who promote and drive reforms within the UN system. The empirical development demonstrates that the swift incorporation of the external pressure into a reform process in the mid-2000s cannot be dissociated from the active support of some UN agencies who have had a common interest in shifting institutional arrangements inside UNAIDS to expand their bureaucratic authority. In conclusion, the paper suggests analyzing reforms within international administrations as social processes driven by both coercion and opportunities