Breast cancer remains a leading cause of cancer-related death within the female population. Immunotherapy is expected to provide additional therapeutic benefits but has met so far limited success. This may be due in part to the poor understanding of immune responses to breast cancer. Although CD4 1 and CD8 1 T lymphocytes infiltrate these tumors, the phenotype and functions of these cells remain ill defined. This study was designed to investigate further about these questions, taking advantage of multiparameter flow cytometry on lymphocytes derived from peripheral blood, solid tumors, metastatic lymph nodes and pleural effusions samples of patients with breast cancer. Results showed that, in addition to conventional CD4 1 and CD8 1 ab T cells, individual tumors and most pleural effusions contained significant fractions of unconventional double positive (DP) CD4 1 CD8 1 ab T cells. These DP T cells displayed the phenotype and cytotoxic potential of effector/memory activated CD8 1 T cells but differed essentially from these cells by a high production of IL-5 and IL-13. The increased frequency of DP T cells in advanced breast cancer and their high lytic potential and original cytokine profile suggest that this T-cell subset may play a specific role in the regulation of immune responses to human breast cancer.