Transforming growth factor beta (TGF-β) is a multifunctional cytokine, whose myriad of functions include its ability to potently suppress the immune system. Because of its ability to negatively modulate the inductive and effector phases of the immune response, TGF-β is thought to contribute to tumor progression and metastases formation. Immunosuppression by tumor-derived TGF-β is increasingly becoming recognized as an important factor in tumor progression and may, in part, explain the low response rates achieved in cancer patients undergoing immunotherapy for their disease. This review will focus on the immunosuppressive role of tumor-derived TGF-β in breast cancer. Due to the paucity of human studies, it will specifically address the actions of tumor-derived TGF-β on cells of the immune system in preclinical animal models, as well as discuss strategies to negate the deleterious effects of TGF-β in order to improve the anti-tumor immune response. © 2003 Elsevier Inc. All rights reserved.