In healthy lungs, branched airways are lined by several epithelial cell types, each of which is important in pulmonary homoeostasis. Little is known about how the differentiation of these cell types is regulated. Wan et al. now report that the winged-helix transcription factor Foxa2 regulates alveolarisation and airway epithelial cell differentiation in the postnatal mouse lung (see p. 953). When the researchers used a conditional deletion approach to delete Foxa2 in developing mouse lungs, they saw airspace enlargement within the lungs of the Fox2a-deleted mice together with hyperplasia of the mucous-secreting goblet cells. This type of hyperplasia is associated with chronic human lung diseases, and the researchers show that FOX2A expression and goblet cell hyperplasia are inversely correlated in patients with such diseases.