Laboratory evidence is presented that estrogens are able to induce the production of numerous growth factors which can act in an autocrine or paracrine fashion in estrogen dependent breast cancer. Estrogen independent tumors can produce these same growth factors constitutively and so escape the need for estrogen stimulation. Growth inhibitory factors such as TGF-beta can also be controlled by estrogens and antiestrogens. It is unclear at present, however, how much of the cytostatic effect of antiestrogens in vivo is explained by the production of growth inhibitors. The overall control of breast cancer growth is mediated by the combined effects of these growth stimulatory and inhibitory factors in both breast stroma and epithelium. Interruption of the action of growth factors and the use of growth inhibitors may provide opportunities for new approaches to the treatment of breast cancer.