Lymphopenia-induced homeostatic proliferation of T cells after autologous hematopoietic stem cell transplantation (HSCT) skews the T cell repertoire by engaging tumor-associated Ags, leading to an induction of antitumor immunity. However, how HSCT alters the immunosuppressive microenvironment in the tumors is unknown. In this study, we first analyzed the kinetics of regulatory T cells (Tregs) in the tumors after syngeneic HSCT. Unexpectedly, the frequency of CD4⁺ cells expressing Foxp3 was increased in the spleens, whereas the frequency was clearly decreased in the tumors after HSCT. The origin of reconstituted CD4⁺ and Foxp3⁺ cells in the tumors was mainly from the expansion of transferred splenic T cells. Then, to examine the mechanism of Treg suppression after HSCT, we isolated CD11c⁺ cells from tumors. A large amount of Treg-inhibitory cytokine IL-6 was secreted from the CD11c⁺ cells in the tumors, but not in the spleens in the recipient mice. Furthermore, to understand what factor affects the activity of CD11c⁺ cells in the tumors after HSCT, we analyzed the expression of various cytokines/chemokines with mouse cytokine Ab arrays, and noticed that VEGF-D concentration was increased in the tumors in the early period after HSCT. The CD11c⁺ cells produced IL-6 in response to VEGF-D stimulation, and an administration of VEGF receptor-3 neutralizing Ab significantly suppressed the production of IL-6 from CD11c⁺ cells accompanied with the increase of Tregs in the tumors of HSCT recipients. Autologous HSCT creates an environment that strongly supports the enhancement of antitumor immunity in reconstituted lymphopenic recipients through the suppression of Tregs.