ExtractIn order to test the role of diffusing capacity in determining the arterial oxygen tension of newborn infants, pulmonary diffusing capacity for carbon monoxide (DLco) was measured in 21 healthy infants ranging in birth weight from 765 to 4,720 g. DLcoin infants without respiratory distress correlated well with lung volume (r= 0.76,P< 0.001). A smaller correlation (r= 0.39,P< 0.01) was obtained between DLcoand arterial oxygen tension. DLcoper unit volume of lung is similar when healthy premature infants, full term infants, and normal adults are compared.The wide range of normal values obtained in resting infants and the lack of correlation with arterial oxygen tensions are similar to observations made in adults.SpeculationDiffusion “block” was not demonstrable in this study, even in infants with mild hypoxemia. These data, in conjunction with other studies reviewed in this report, suggest that lung volume plays a critical role in the level of arterial oxygen tension in newborn infants.