New vessel formation is a prerequisite for the growth of a tumour mass. Growing evidence suggests that endothelial progenitor cells circulate in the blood and participate in that process. The purpose of the present study was ultrastructural and electron microscopic immunocytochemical examination of capillary blood vessels in human glioma. The results showed striking morphological changes in these vessels. Our observations indicate that tumours build vessels by cooption of pre-existing vasculature and de novo recruitment of endothelial progenitor cells. Immature endothelial cells characterized by fibrils in the cytoplasm and Flk-1 positive immunoreactivity were observed as small clusters or luminally localized individual endothelial progenitor cells that participate in intussusceptive vessel growth. This observation indicates that the tumour microenvironment determines biological and functional attributes of endothelial progenitor cells.