Although the therapeutic role of angiogenesis target therapy has been approved in cancer treatment, the way to optimize the dose of angiogenesis inhibitors remains to be determined because of the lack of reliable surrogate markers of tumor angiogenesis. Shaked et al. reported that the levels of circulating endothelial progenitor cells (EPC), which contribute to the tumor vessel formation, reflect the anti-tumor efficacy of anti-angiogenesis regimens (9,10). Growing evidence suggests that the levels of circulating EPC reflect the response to chemotherapy both in animal model and clinical trial (11-13). Thus, circulating EPC can be used as a marker for optimizing and monitoring the anti-angiogenesis therapy including angiogenesis inhibitors and chemotherapy.