It is well known that tumor-infiltrating lymphocytes (TILs) and, to a lesser extent, peripheral blood lymphocytes from patients with advanced-stage cancer have a poor immune response. Regulatory T cells (T-regs), characterized by coexpression of CD4 and CD25 markers, can inhibit the immune response mediated by CD4+/CD25- and CD8+ T cells. In the present study, we evaluated the prevalence of T-regs in peripheral blood and TILs in patients with gastric and esophageal cancers. The population of CD4+/CD25+ cells as a percentage of total CD3+ cells was evaluated by flow cytometric analysis with triple-color staining. To assess the functional activity of CD4+/CD25+ cells, CD4+/CD25+ or CD4+/CD25- cells were purified from peripheral blood mononuclear cells with magnetic beads. The cytokine production [interleukin (IL)-10 and IFN-gamma] from the CD4+/CD25+ cells in response to anti-CD3 stimulation was evaluated. Also, the antiproliferative function of CD4+/CD25+ cells was measured by evaluating the proliferative activity of CD4+/CD25- cells in response to anti-CD3 plus anti-CD28 in the presence of autologous CD4+/CD25+ cells. The prevalence of peripheral blood CD4+/CD25+ cells in both gastric (n = 20; 14.2 +/- 4.9%) and esophageal cancer patients (n = 10; 19.8 +/- 6.9%) was significantly higher than that in healthy donors (n = 16; 7.2 +/- 2.1%). The population of CD4+/CD25+ cells in the TILs of gastric cancer patients with advanced disease (19.8 +/- 4.5%) was significantly higher than that in TILs of patients with early-stage disease (4.8 +/- 2.1%) or that in intraepithelial lymphocytes of normal gastric mucosa (4.0 +/- 1.2%). As a functional consequence, CD4+/CD25+ cells did not produce IFN-gamma, whereas CD4+/CD25- cells secreted IFN-gamma. Moreover, CD4+/CD25+ cells produced large amounts of IL-10, whereas CD4+/CD25- cells secreted little IL-10. The proliferation of CD4+/CD25- cells was inhibited in the presence of CD4+/CD25+ cells in a dose-dependent manner, confirming that CD4+/CD25+ has an inhibitory activity corresponding to T-regs. The populations of CD4+/CD25+ T-regs in peripheral blood and TILs in patients with gastric and esophageal cancers were significantly higher in comparison with those in healthy donors or normal mucosa.