The morphology of postnatal human lung development and growth has been investigated by light and by scanning and transmission electron microscopy in seven children dying from non-respiratory causes and aged between 26 days and 64 months. The findings are compared with those of adult human lungs and are discussed in relation to the postnatal lung development in other species, particularly rodents. Within the first 1 1/2 postnatal years lung parenchyma undergoes a substantial structural remodeling due to bulk alveolar formation and to the restructuring of septal morphology. At one month alveolar formation appears to be well under way: The human lung is comparable then to a rat lung aged one week. In the parenchyma, numerous short and blunt tissue ridges, so-called secondary septa, subdivide the peripheral airspaces into an increasing number of still very shallow alveoli. The parenchymal septa present during and after alveolization are immature: they contain a double capillary network with a central, highly cellular sheet of connective tissue. The septal maturation sets in a few months after birth and consists of a reduction in the interstitial tissue mass and a complex process of capillary remodeling. Both alveolization and parenchymal maturation progress rapidly: by 6 months the lung has taken a big step towards maturity. By 1 1/2 years most septa show the adult structure where a single capillary network interwoven with connective tissue strands stabilizes the interalveolar wall. After the septal restructuring, lung development is considered complete, and the lung enters a period of normal growth that lasts until adulthood. From our observations we conclude that postnatal human lung development is made of two overlapping stages: (a) the alveolar stage, which starts in late fetal life and lasts to about 1-1 1/2 years, and (b) a stage of microvascular maturation, thought to extend from the first months after birth to the age of 2-3 years.