The mechanisms involved in the regulation of pulmonary vascular endothelial cells to replace aged or injured cells remains poorly understood, although differences in proliferative potential between the microvascular and macrovascular endothelium are well described. The presence of resident pulmonary vascular endothelial progenitor cells in rats and mice has been recently reported. These resident endothelial progenitor cells display clonal proliferative potential, restricted expression of cell surface molecules to those typical of lung endothelium, and in vivo vessel-forming ability upon transplantation into recipient animals. The rat pulmonary microvascular endothelium is enriched in resident progenitor cells, with the highest proliferative potential compared with the pulmonary macrovascular endothelium. Preliminary evidence suggests that resident endothelial progenitor cells are present in the human lung vasculature, but whether differences in enrichment of the progenitors in various pulmonary vascular beds exist remains to be determined.