CD4⁺ Treg cells expressing the transcription factor FOXP3 (forkhead box P3) are abundant in tumor tissues and appear to hinder the induction of effective antitumor immunity. A substantial number of T cells, including Treg cells, in tumor tissues and peripheral blood express C-C chemokine receptor 4 (CCR4). Here we show that CCR4 was specifically expressed by a subset of terminally differentiated and most suppressive CD45RA⁻FOXP3^hiCD4⁺ Treg cells [designated effector Treg (eTreg) cells], but not by CD45RA⁺FOXP3^loCD4⁺ naive Treg cells, in peripheral blood of healthy individuals and cancer patients. In melanoma tissues, CCR4⁺ eTreg cells were predominant among tumor-infiltrating FOXP3⁺ T cells and much higher in frequency compared with those in peripheral blood. With peripheral blood lymphocytes from healthy individuals and melanoma patients, ex vivo depletion of CCR4⁺ T cells and subsequent in vitro stimulation of the depleted cell population with the cancer/testis antigen NY-ESO-1 efficiently induced NY-ESO-1–specific CD4⁺ T cells. Nondepletion failed in the induction. The magnitude of the responses was comparable with total removal of FOXP3⁺ Treg cells by CD25⁺ T-cell depletion. CCR4⁺ T-cell depletion also augmented in vitro induction of NY-ESO-1–specific CD8⁺ T cells in melanoma patients. Furthermore, in vivo administration of anti-CCR4 mAb markedly reduced the eTreg-cell fraction and augmented NY-ESO-1–specific CD8⁺ T-cell responses in an adult T-cell leukemia-lymphoma patient whose leukemic cells expressed NY-ESO-1. Collectively, these findings indicate that anti-CCR4 mAb treatment is instrumental for evoking and augmenting antitumor immunity in cancer patients by selectively depleting eTreg cells.