Circulating endothelial cells (CECs) are increased in the peripheral blood of cancer patients and appear to be a marker of tumor related angiogenesis. Studies suggest that CECs increase in the setting of progressive disease and decrease in response to treatment with an antiangiogenic agent. Although current data evaluating CECs in RCC is limited, there is significant interest in CEC evaluation as a marker of treatment response given the majority of current FDA approved RCC therapies target tumor angiogenesis.