Circulating endothelial cell (CEC) and progenitor (CEP) number and viability are modulated in various pathological conditions including cancer. There is increasing evidence showing that CEC and CEP play a role in cancer progression and metastasis in different animal models. At the clinical level, emerging data support that CEC and CEP kinetics and viability might predict the efficacy on anticancer drug combinations that include antiangiogenic agents. On the basis of these observations, CEC and CEP measurements have attractive potential diagnostic and therapeutic applications for malignant diseases.