Angiogenesis is a critical determinant of alveolarization, which increases alveolar surface area and pulmonary capillary blood volume in infants; however, our understanding of this process is very limited.The purpose of our study was to measure the membrane (DM) and pulmonary capillary (VC) components of pulmonary diffusion to carbon monoxide (DL,CO) in healthy infants and toddlers, and evaluate whether these components were associated with pro-angiogenic circulating hematopoietic stem/progenitor cells (pCHSPCs) early in life.Healthy subjects (N=21; 11 males), 3-25 months of age, were evaluated. DL,CO was measured under normoxic and hyperoxic conditions, and DM and VC were calculated. From 1-ml venous blood, CHSPCs were quantified by multiparametric flow cytometry.DM and VC increased with increasing body length; however, membrane resistance as a fraction of total resistance to pulmonary diffusion remained constant with somatic size. In addition, DL,CO and VC, but not DM, increased with an increasing percentage of pCHSPCs.The parallel increase in the membrane and vascular components of pulmonary diffusion is consistent with alveolarization during this period of rapid lung growth. In addition, the relationship between pCHSPCs and pulmonary capillary blood volume suggest that pro-angiogenic cells may contribute to this vascular process.