CD105 (endoglin), a receptor for transforming growth factor (TGF) โค1 and โค3 in vascular endothelial cells, is highly up-regulated in blood vessels of tissues where neovascularisation occurs. It modulates endothelial-mesenchymal signalling and is essential for angiogenesis. Indeed, CD105 knockout mice die from malvascularisation by 11.5 day p.c. In the present study CD105, TGFโค1 and CD105/TGFโค1 complexes were quantified in plasma samples from 77 healthy individuals and 92 patients with early stage breast cancer prior to any treatment. When compared with normal controls, both CD105 and CD105/TGFโค1 complex levels were significantly elevated in breast cancer patients, whereas TGFโค1 levels were lower in cancer patients. The most important finding to emerge was that CD105 levels were significantly increased in patients who developed distant metastasis compared with disease-free patients. While there was no significant difference between CD105 levels in controls compared to diseasefree patients, it was significantly higher in patients with metastatic disease. Thus patients who had died following local relapse or distant metastases possessed the highest levels of CD105. Neither CD105/TGFโค1 complex nor TGFโค1 levels correlated with tumour progression. Our data indicate that CD105 might be a valuable novel angiogenic marker for identifying breast cancer patients who are at high risk of developing metastasis.