To detect endothelial progenitor cells in human umbilical veins and isolated endothelial cells, the authors examined protein and mRNA expression levels of cell surface markers for endothelial progenitor cells in human umbilical veins before and after trypsin treatment and at different passages of the isolated endothelial cells. CD133(+) (2.14 +/- 0.57 per mm) and KDR(+) (35.74 +/- 8.28 per mm) cells were observed in the intima of umbilical veins. The amounts of CD133(+), KDR(+), CD34(+), and CD105(+) cells decreased in the intima after trypsin treatment, whereas the percent of CD133(+) and KDR(+)cells in the media did not change significantly. Moreover, similar protein and mRNA expression levels of CD133 and KDR were detected in the umbilical veins before and after trypsin treatment. In the isolated cells from umbilical veins, the percent of CD133(+) and CD34(+) cells in P1 was 3.43% +/- 3.85%, which was higher than those in P3 (0.17% +/- 0.21%, p = 0.005) and P6 (0.14% +/- 0.18%, p = .001). The mRNA expression levels of CD133 and CD105 were down-regulated in later passages compared to those in P1, whereas the expression level of KDR was up-regulated in late passages. Thus it is suggested that endothelial progenitor cells reside in the distinct zone (e.g., initma and media) of human umbilical veins, and retain the capacity of differentiation to endothelial cells in vitro.