According to the current view the formation of new alveolar septa from preexisting ones ceases due to the reduction of a double to a single layered capillaries network inside the alveolar septa (microvasculature maturation postnatal days 14-21 in rats). We challenged this view by measuring stereologically the appearance of new alveolar septa and by studying the alveolar capillary network in 3D-visualizations obtained by high resolution synchrotron radiation X-ray tomographic microscopy. We observed that new septa are formed at least until young adulthood (rats, days 4-60) and that roughly half of the new septa are lifted off of mature septa containing single layered capillary networks. At the basis of newly forming septa we detected a local duplication of the capillary network. We conclude that new alveoli may be formed in principle at any time and at any location inside the lung parenchyma and that lung development continues into young adulthood. We would like to define two phases during developmental alveolarization. Phase one (days 4-21), lifting off of new septa from immature preexisting septa and phase two (days 14-young adulthood), formation of septa from mature preexisting septa. Clinically, our results ask for precautions using drugs influencing structural lung development during both phases of alveolarization.