['Granulocyte colony-stimulating factor (G-CSF) is known to mobilize endothelial progenitor cells (EPCs) from bone marrow. EPCs reportedly promote neovascularization and participate in the repair of lung structure in adult animals.', 'We tested the hypothesis that G-CSF contributes to alveolar growth by increasing the production of angiogenic growth factor in the lungs of hyperoxia-exposed neonatal mice.', 'Neonatal mice were exposed to hyperoxia (80%) or room air (RA) for 7 days and treated with G-CSF (50 μg/kg/day) or vehicle for 5 days. Blood was subjected to flow cytometry to gate for CD45(dim/-)/Sca-1(+)/CD133(+)/vascular endothelial growth factor (VEGF) receptor-2 (VEGFR2) to define the EPC population at day 7.', 'The percentage of EPCs in the peripheral blood and VEGF and VEGFR2 levels in the lungs of neonatal mice exposed to hyperoxia were significantly reduced compared to those of mice kept in RA. G-CSF significantly increased EPCs in the peripheral blood, and VEGF and VEGFR2 levels in the lungs of both mice exposed to hyperoxia and mice kept in RA. G-CSF restored alveolarization inhibited by hyperoxia without altering normal alveolarization under RA.'] Does granulocyte colony-stimulating factor reduce hyperoxia-induced alveolarization inhibition by increasing angiogenic factors?