Interleukin (IL)-7 is a critical cytokine regulating T-lymphocyte development, regeneration, and function. This study analyzes the endogenous IL-7 production in HIV-infected patients receiving highly active antiretroviral treatment (HAART). Plasma levels of IL-7 were measured by enzyme-linked immunosorbent assay (ELISA) in 11 patients with untreated advanced HIV disease, in 8 patients who successfully responded to HAART, and in 9 individuals with virological and immunological treatment failure. We found that in the patients with advanced HIV disease and no treatment IL-7 concentrations were elevated and were inversely related to both CD4 + and CD8 + T-cell counts. When IL-7 was assessed in treated patients, this cytokine was below the detection limit of the assay in all participants who responded to HAART. On the contrary, patients with evidence of HAART failure had increased concentrations of IL-7 that were comparable to those found in the untreated group with progressive disease. These data suggest that IL-7 may play a role in the immune reconstitution of T-cells during HIV infection, especially in the context of potent antiretroviral treatments.