Exposure of preterm infants to hyperoxia impairs vascular growth, contributing to the development of bronchopulmonary dysplasia and retinopathy of prematurity. Disruption of vascular endothelial growth factor (VEGF)-nitric oxide (NO) signaling impairs vascular growth. Endothelial progenitor cells (EPCs) may play an important role in vascular growth. Endothelial colony forming cells (ECFCs), a type of EPC, from human preterm cord blood (CB) are more susceptible to hyperoxia-induced growth impairment than term ECFCs. Therefore, we hypothesized that hyperoxia disrupts VEGF-NO signaling and impairs growth in preterm ECFCs, and that exogenous VEGF or NO preserves growth in hyperoxia. Growth kinetics of preterm CB-derived ECFCs (gestational ages, 27-34 weeks) were assessed in room air (RA) and hyperoxia (40-50% oxygen) with or without VEGF, NO, or N-Nitro-L-arginine (LNA). VEGF, VEGF receptor-2 (VEGFR-2), and endothelial NO synthase (eNOS) protein expression and NO production were compared. In comparison with RA controls, hyperoxia significantly decreased growth, VEGFR-2 and eNOS expression, and NO production. VEGF treatment restored growth in hyperoxia to values measured in RA controls and significantly increased eNOS expression in hyperoxia. NO treatment also increased growth in hyperoxia. LNA treatment inhibited VEGF-augmented growth in RA and hyperoxia. We conclude that hyperoxia decreases growth and disrupts VEGF-NO signaling in human preterm ECFCs. VEGF treatment restores growth in hyperoxia by increasing NO production. And NO treatment also increases growth during hyperoxia. Exogenous VEGF or NO may protect preterm ECFCs from the adverse effects of hyperoxia and preservation of ECFC function may improve outcomes of preterm infants.