Background Endoglin (CD105) is a co-receptor for TGF-beta, is expressed by human vascular endothelial cells, and plays a major role in angiogenesis. Materials and methods Pretreatment EDTA plasma from 224 metastatic breast cancer patients enrolled in a phase III 2nd-line hormone therapy trial and 50 control subjects were assayed for endoglin using an ELISA. Results The female control group (n = 50) plasma endoglin upper limit of normal was defined as the mean + 2 SD (8.7 ng/ml). The breast cancer patient plasma endoglin was 6.40 +/- 2.23 ng/ml (range 3.00-19.79 ng/ml). Elevated plasma endoglin levels were detected in 26 of 224 patients (11.6%). Patients with elevated plasma endoglin had a reduced clinical benefit rate (CR + PR + Stable) (15 vs. 42%) (P = 0.01) to hormone therapy. TTP was shorter for patients with elevated plasma endoglin, but did not reach statistical significance (P = 0.2). Patients with elevated plasma endoglin had decreased overall survival (median 645 vs. 947 days) (P = 0.005). Conclusion Elevated pretreatment plasma endoglin levels predicted for decreased clinical benefit and a shorter overall survival in metastatic breast cancer patients treated with 2nd-line hormone therapy.