CD25+CD4+ regulatory T cells (Treg) regulate peripheral self-tolerance and possess the ability to suppress antitumor responses, which may explain the poor clinical response of cancer patients undergoing active immunization protocols, and provides the rationale for neutralizing Treg cells in vivo to strengthen local antitumor immune responses. Because interleukin-2 (IL-2) mediates tumor regression in about 15% of treated patients but simultaneously increases Treg cells, we hypothesized that transient elimination of Treg cells will enhance the clinical effectiveness of IL-2 therapy. In the current study, 5 patients with metastatic melanoma who were refractory to prior IL-2 received a lymphodepleting preparative regimen followed by the adoptive transfer of autologous lymphocytes depleted of CD25+ Treg cells and high-dose IL-2 administration. CD25+ cells were eliminated from patient leukapheresis samples using a clinical-grade, large-scale immunomagnetic system, leaving CD8+ and CD25−CD4+ T cells intact. In the early aftermath of CD25+ Treg cell-depleted cell infusion, CD25+FOXP3+ CD4+ Treg cells rapidly repopulated the peripheral blood of treated patients with 18% to 63% of CD4+ T cells expressing FOXP3. Recovering CD25+CD4+ T cells exhibited suppressive activity against CD25−CD4+ effector T-cell proliferation in vitro. No patient experienced objective tumor regression or autoimmunity. Our results indicate that in vivo transfer of autologous CD25-depleted mononuclear populations to lymphopenic patients in combination with high-dose IL-2 is not sufficient to mediate prolonged reduction of Treg cells after IL-2 administration.