Accumulating evidence emphasizes the emerging role of circulating endothelial cells (CECs) and endothelial progenitor cells (EPCs) in tumor angiogenesis as surrogate markers and in the efficacy of anti-angiogenic therapies in breast cancer (22-27). Furstenberger et al. (22) reported that CECs were significantly elevated in breast cancer patients and decreased during chemotherapy (anthracycline and/or taxane based). However, EPCs (CD34+/VEGFR-2+) as well as their progenitor cell population CD133+/CD34+ and the population of CD34+ stem cells increased together with VEGF, erythropoietin and angiopoietin-2 levels. Their data suggest that chemotherapy reduces mature CECs, while mobilizing the EPC population. Using real-time PCR and flow cytometry, they also showed that CD146, an endothelial cell specific antigen, was significantly increased in newly diagnosed breast cancer patients compared to healthy controls (23). Other studies also reported increased circulating EPCs in breast cancer patients in addition to CECs (24,25). In another study, circulating EPCs were not increased in cancer patients despite the high plasma VEGF levels (26). Another interesting aspect is that Mancuso et al (27) showed that CEC kinetics correlate with progression-free and overall survival but not circulating progenitor cells in metastatic breast cancer.