Regulatory T cells (Treg) that prevent autoimmune diseases by suppression of self-reactive T cells may also suppress the immune response against cancer. In mice, depletion of Treg by Ab therapy leads to more efficient tumor rejection. Treg-mediated suppression of antitumor immune responses may partly explain the poor clinical response to vaccine-based immunotherapy for human cancer. In this study, we measured the prevalence of Treg that coexpress CD4 and CD25 in the PBLs, tumor-infiltrating lymphocytes, and regional lymph node lymphocytes from 65 patients with either pancreas or breast cancer. In breast cancer patients (n = 35), pancreas cancer patients (n = 30), and normal donors (n = 35), the prevalence of Treg were 16.6% (SE 1.22), 13.2% (SE 1.13), and 8.6% (SE 0.71) of the total CD4+ cells, respectively. The prevalence of Treg were significantly higher in breast cancer patients (p < 0.01) and pancreas cancer patients (p < 0.01) when compared with normal donors. In tumor-infiltrating lymphocytes and lymph node lymphocytes, the Treg prevalence were 20.2% (SE 3.93) and 20.1% (SE 4.3), respectively. Treg constitutively coexpressed CTLA-4 and CD45RO markers, and secreted TGF-β and IL-10 but did not secrete IFN-γ. When cocultured with activated CD8+ cells or CD4+25- cells, Treg potently suppressed their proliferation and secretion of IFN-γ. We conclude that the prevalence of Treg is increased in the peripheral blood as well as in the tumor microenvironment of patients with invasive breast or pancreas cancers. These Treg may mitigate the immune response against cancer, and may partly explain the poor immune response against tumor Ags.