CD4(+) regulatory T cells frustrate productive tumor immune surveillance and represent an obstacle for cancer immunotherapy. In mice, anti-CD25 antibody is an effective method of depleting CD25(+) FOXP3(+) T regulatory cells (Tregs) in vivo and enhancing cancer immunity. Here, we propose the use of the anti-CD25 monoclonal antibody daclizumab for the depletion of Tregs in cancer patients. In early results from an ongoing clinical trial, a single intravenous infusion of daclizumab in patients with metastatic breast cancer is associated with a marked and prolonged elimination of CD25(+) FOXP3(+) Tregs in peripheral blood. When a cancer antigen peptide vaccine is administered during the daclizumab-induced Treg nadir, effective generation of cytotoxic T lymphocytes has been observed, including those specific for neo-antigens, such as cytomegalovirus peptide used as an immunological control. If confirmed in additional patients, these observations suggest that daclizumab may be an effective and available therapeutic agent for Treg modulation in patients with cancer.