B reast cancer frequently metastasizes to the skeleton, and the associated bone destruction is mediated by the osteoclast. Growth factors, including transforming growth factor-β (TGF-β), released from bone matrix by the action of osteoclasts, may foster metastatic growth. Because TGF-β inhibits growth of epithelial cells, and carcinoma cells are often defective in TGF-β responses, any role of TGF-β in metastasis is likely to be mediated by effects on the surrounding normal tissue. However, we present evidence that TGF-β promotes breast cancer metastasis by acting directly on the tumor cells. Expression of a dominant–negative mutant (TβRIIΔcyt) of the TGF-β type II receptor rendered the human breast cancer cell line MDA-MB-231 unresponsive to TGF-β. In a murine model of bone metastases, expression of TβRIIΔcyt by MDA-MB-231 resulted in less bone destruction, less tumor with fewer associated osteoclasts, and prolonged survival compared with controls. Reversal of the dominant–negative signaling blockade by expression of a constitutively active TGF-β type I receptor in the breast cancer cells increased tumor production of parathyroid hormone–related protein (PTHrP), enhanced osteolytic bone metastasis, and decreased survival. Transfection of MDA-MB-231 cells that expressed the dominant–negative TβRIIΔcyt with the cDNA for PTHrP resulted in constitutive tumor PTHrP production and accelerated bone metastases. These data demonstrate an important role for TGF-β in the development of breast cancer metastasis to bone, via the TGF-β receptor–mediated signaling pathway in tumor cells, and suggest that the bone destruction is mediated by PTHrP.