It has been suggested that immunosuppressive cytokines such as transforming growth factor beta (TGF-beta) and interleukin 10 play an important role in the induction and/or maintenance of regulatory T-cells (Tregs) in patients with cancer. In the present study, whether or not vascular endothelial growth factor (VEGF) contributes to the induction and/or maintenance of Tregs was examined, because of experience with a patient in whom a positive correlation between VEGF concentration and the percentage of Tregs (% Tregs) among the total CD4(+) T-cells in the pleural effusion was found during dendritic cell activated lymphocyte therapy. CD4(+)CD25(high) T-cells were estimated as Tregs in the present study. In an in vitro experimental system, VEGF-containing malignant effusions increased the % Tregs in autologous peripheral blood mononuclear cells (PBMCs), which could be suppressed by the addition of a humanized monoclonal anti-VEGF antibody (bevacizumab [Avastin]). When VEGF-producing hepatic carcinoma cells were mix-cultured with PBMCs, the % Tregs increased and this increase was also suppressed by the addition of bevacizumab. Whether or not bevacizumab can affect the % Tregs of PBMCs in patients with colon cancer was also examined. Three out of four patients showed a significant decrease of the % Tregs after intravenous injection of bevacizumab. Interestingly, the expression of VEGF receptor-2 (VEGFR-2) was higher in Tregs than in other CD4(+) T-cells. Taken together, the data presented here indicate a contribution of VEGF to induction and/or maintenance of Tregs in patients with cancer.