The mechanisms through which regulatory T cells accumulate in lymphoid organs of tumor-bearing hosts remain elusive. Our experiments indicate that the accumulation of CD4⁺CD25⁺ regulatory T cells (T reg cells) expressing FoxP3 and exhibiting immunosuppressive function originates from the proliferation of naturally occurring CD25⁺ T cells and requires signaling through transforming growth factor (TGF)–β receptor II. During tumor progression, a subset of dendritic cells (DCs) exhibiting a myeloid immature phenotype is recruited to draining lymph nodes. This DC subset selectively promotes the proliferation of T reg cells in a TGF-β–dependent manner in mice and rats. Tumor cells are necessary and sufficient to convert DCs into regulatory cells that secrete bioactive TGF-β and stimulate T reg cell proliferation. In conclusion, tumor expansion can stimulate T reg cells via a specific DC subset.