A workshop entitled "Regulatory T cells (T(reg)) and HIV/AIDS" was held in Cincinnati, OH, September 7-8, 2006. This workshop was the first completely dedicated to T(reg) in HIV infection, and gathered investigators working on different aspects of T(reg) biology, and on HIV pathogenesis. We report here the major topics of discussion of this workshop, the goal of which was to summarize what is known and not known about the role of T(reg) in HIV immune responses and pathogenesis; and to foster discussion on the means of manipulating T(reg) in HIV-infected subjects. Workshop participants also debated the research priorities in the field, which emerged as follows: (1) to arrive at a consensus on T(reg) definition and to standardize assays aimed at characterizing T(reg) number and function; (2) to study T(reg) biology in tissues, notably in mucosal tissues, and at different stages of infection; (3) to examine T(reg) function in vivo in animal models, as well as to test strategies to target or modulate T(reg) cell function in these models; (4) to investigate the effect of viral factors on T(reg) biology, and conversely to study whether T(reg) activity affects the level of HIV replication; (5) to conduct longitudinal studies of T(reg) number and function; and (6) to determine the effect of coinfections on T(reg) biology.