Results obtained from preclinical studies have shown that endothelial progenitor cells (EPCs) play a crucial role in tumor growth and metastasis. In the clinic, EPCs are present in the peripheral blood of cancer patients in higher numbers than in healthy subjects. These cells are mobilized from the bone marrow compartment to the periphery in response to certain cytokines and growth factors. Growing body of evidence suggests that following acute cytotoxic drug therapy levels of circulating EPCs (CEPs) can change significantly in both mouse and human. These changes may predict the efficacy of some anticancer drug treatments. Therefore, the validation and standardization of a procedure to detect CEPs and monitor their kinetic is an important step towards the use of such cells as a possible biomarker to predict clinical outcome. In this chapter, we describe a flow cytometry technique to detect CEPs obtained from human blood specimens stored in both fresh and frozen conditions.