As a natural ligand for CD4, IL-16 has been shown to preferentially induce migration in Th1 cells, and, in long-term cultures with IL-2, IL-16 facilitates the expansion of CD4+ CD25+ cells. In addition, IL-16 has an immunomodulatory role in asthmatic inflammation, as exogenous administration significantly reduces inflammation and airway hyperreactivity. The mechanism for this, however, is not clear. Based on its functional characteristics and potential immunomodulatory role, we investigated the ability of IL-16 to recruit and influence the development of T regulatory (Treg) cells. We now demonstrate that IL-16 preferentially induces migration in a CD25+ CTLA-4+ human T cell subset and that responding cells produce IFNγ and TGFβ but not IL-10. These cells are relatively unresponsive to antigenic stimulation and can suppress proliferation and IL-5, but not IFNγ, production by autologous T cells. We further demonstrate that IL-16-recruited cells are enriched for Forkhead box P3 (Foxp3). In addition, we find that IL-16 stimulation may facilitate de novo induction of Foxp3+ Treg cells, because the stimulation of FoxP3-negative T cells for 48 h results in the expression of FoxP3 mRNA and protein. These data indicate that at sites of inflammation IL-16 may contribute to selective Treg cell expansion through the preferential induction of a migratory response from existing Treg cells, as well as by the induction of de novo generation of FoxP3+ cells. These findings offer a potential mechanism for the immunosuppressive effects of IL-16 seen in Th2-mediated inflammation.