Postnatal lung development is not well characterized in mice, especially the time point when alveolarization is completed. Using the total length and the length density of the free septal edge as measured for the formation of new septa, we followed alveolarization throughout postnatal lung development (days 2–125). Furthermore, the alveolar surface area was estimated. The formation of new septa was observed until day 36. Approximately 10% of the septa present in adult mice were formed prenatally by branching morphogenesis, ∼50% were generated postnatally before and ∼40% after maturation of the alveolar microvasculature. Approximately 5% of the alveolar surface area present during adulthood was present before alveolarization started, ∼55% was formed during alveolarization (days 4–36) and ∼40% afterward due to growth processes. We conclude that alveolarization continues until young adulthood and that the maturation of the alveolar microvasculature does not preclude further alveolarization. Developmental Dynamics 237:2108–2116, 2008. © 2008 Wiley‐Liss, Inc.