An increase in the number of circulating endothelial cells (CEC) and of bone marrow-derived endothelial progenitor cells (EPC) in the peripheral blood is associated with vascular injury, repair and neovascularization. The phenotype and number of CEC may serve as diagnostic or prognostic parameters of vascular injury and tumour growth. An increase in the number of EPC may reflect repair of ischaemic vascular injury, a finding which has resulted in the initiation of clinical cardiovascular pilot trials using cell therapy. However, there is no consensus on the exact phenotype of the EPC and haematopoietic stem cells (HSC) and therefore the best candidate cell for transplant has not been established. Although the use of peripheral blood stem cells following mobilization, or of ex vivo-expanded cells, may improve EPC-mediated vascular graft endothelialization or tissue vascularization, sustained EPC-induced neovascularization still needs to be proven. Flow cytometric characterization, in combination with functional assays, will further elucidate the phenotype of the CEC and EPC, thereby providing reliable detection to appreciate their role in vascular diseases and cancer and to evaluate and, if possible, improve their therapeutic potential.