Lungs of 482 infants were examined by artificial inflation. The pattern of aeration is recognized best when expansion is incomplete. It varies strikingly with maturity: inpreviable fetuses (approximately 1,000 gm or less) and in mature infants (above 2,000 to 2,500 gm) air is distributed at random. In viable premature infants with a birth weight between these two groups, aeration of the bronchi and respiratory bronchioles without expansion of the alveoli (atelectasis of prematurity) is accomplished with relative ease, whereas opening of the alveoli requires much higher pressure. In infants born too small for their gestational age, the pattern of expansionfollows gestational age rather than birth weight. If the normal surface active lining of the air spaces is inactivated or absent, the stability of expansion is reduced and extensive collapse occurs at each expiration. This favors in the lungs of premature infants the recurrence, and eventually persistence of atelectasis of prematurity. Expansion by liquid media shows that the architecture of these atelectatic lungs is normal. The tendency toward atelectasis of prematurity is a function of prematurity, and is independent of the formation of hyaline membranes.