During the last decade, multiple techniques have been developed to isolate and quantify human endothelial progenitor cells (EPCs). In parallel, a number of studies have applied these methodologies to investigate the number and function of circulating EPCs in adult diseases characterized by vascular dysfunction. However, very little is known about different subtypes of EPCs during gestation, during the neonatal age or in neonatal diseases. Initial evidence supports the hypothesis that circulating angiogenic cells may play an important role during development, and attention has particularly focused in clarifying the function of EPCs in lung vascular development, and the role of the impairment of EPC mobilization and homing in hyperoxia-induced lung injury characteristic of bronchopulmonary dysplasia. Among different subtypes of EPCs, both the role of angiogenic mononuclear cells (triple-positive CD34+CD133+VEGFR-2+ cells and colony forming unit-Hill cells) and endothelial colony forming cells (ECFCs) in physiological vascular development and during neonatal diseases need to be elucidated. A better understanding of EPC biology during gestation, during the neonatal age and in preterm infants will unravel the pathologic basis of bronchopulmonary dysplasia and other preterm and term neonatal diseases characterized by a prominent defect in vascular growth, including retinopathy of prematurity and persistent pulmonary hypertension of the newborn.