Chronic immune activation is the leading event in the pathogenesis of HIV-1 infection and is associated with depletion of CD4+ T cells and Th1/Th2 imbalance. The role of Tregs in HIV infection is still controversial as these cells may control both immune activation and HIV-specific T cell responses. Aim of the present study was to correlate the degree of immune activation with FOXP3 expression and production of IL-2 and IL-10. Peripheral blood was obtained from 10 HIV-1-infected subjects with sustained response to HAART. Four cellular fractions were purified and simultaneously used in further experiments: PBMCs, CD4+CD45RO+, CD4+CD45RO+CD25- and CD4+CD45RO+CD25+. The level of immune activation was measured both by flow cytometry and radiometry as short-term 1-hour spontaneous lymphocyte proliferation; IL-2 and IL-10 production were determined by ELISA in supernatants of cultures non-stimulated or stimulated with PHA and p24 antigen. FOXP3 mRNA expression was detected by RT-PCR. A group of healthy subjects was used as control. Statistical analysis was performed by SPSS. The level of immune activation (total CD25 expression) was found correlated to both CD4+ and Tregs (CD4+CD25high+; FOXP3mRNA) and it was found higher in HIV-1 infected subjects in comparison to the healthy control group. The highest SLP-1 h was measured in the CD4+CD4+CD45RO+ CD25- population (T reg depleted), suggesting a role of Tregs in controlling immune activation during HIV-1 infection. In addition, CD4+ cells from HIV-infected individuals remained responsive to the negative regulation by Tregs (measured as IL-10 production) both spontaneously and upon stimulation. It is worth noting, however, that total PBMCs of these patients also responded well to specific antigen (p24) stimulation even though IL-2 release was impaired. The results obtained suggest that Tregs significantly influence the level of immune activation. At the same time, their function appears to be dependent on IL-2 production by the remaining T cells. These findings suggest that the control of Th1/Th2 balance could be an approach to immune-based therapy for HIV infection.