The expression of c-kit and its ligand, stem cell factor (SCF), in developing human lung tissue was investigated by immunohistochemistry. Twenty-eight human fetal lungs [range: 13 to 38 gestational weeks, (GW)] and 12 postnatal lungs (range: 1 to 79 years old) were evaluated. We identified c-kit⁺ cells in the lung mesenchyme as early as 13 GW. These mesenchymal c-kit⁺ cells in the lung did not express mast cell tryptase or α-smooth muscle actin. However, these cells did express CD34, VEGFR2 and Tie-2, indicating their endothelial lineage. Three-dimensional reconstructions of confocal laser scanning images revealed that c-kit⁺ cells displayed a closed-end tube formation that did not contain hematopoietic cells. From the pseudoglandular to the canalicular phase, c-kit⁺ cells appeared to continuously proliferate, to connect with the central pulmonary vessels and finally to develop the lung capillary plexus. The spatial distribution of c-kit and SCF positive cells was also demonstrated, and these cells were shown to be in close association. Our results suggest that c-kit expression the early fetal lungs marks a progenitor population that is restricted to the endothelial lineage. This study also suggests the potential involvement of c-kit signaling in lung vascular development.