Operational parallels in overall mechanisms of three-dimensional patterning of vertebrate organs are becoming increasingly apparent. Many key mediators, such as FGFs, BMPs, and sonic hedgehog, participate in organization of a number of organs, including the lungs, which exhibit a defined proximodistal (P-D) polarity. Recently, Wnt5a a member of the wingless family of signaling molecules involved in cell proliferation, differentiation, and organogenesis, was shown to underlie the outgrowth and P-D morphogenesis of the vertebrate limb. In the current study, we show that Wnt5a is expressed in the mouse lung and plays an important role in lung distal morphogenesis. Analysis of the mutant phenotype in mice carrying a targeted disruption of the Wnt5a locus shows distinct abnormalities in distal lung morphogenesis as manifested by distinct truncation of the trachea and overexpansion of the distal respiratory airways. In the face of deleted WNT5a activity, both epithelial and mesenchymal cell compartments of the Wnt5a(−/−) lungs exhibit increased cell proliferation. The overall architecture of the mutant lungs is characterized by overexpansion of the distal airways and inhibition of lung maturation as reflected by persistence of thickened intersaccular interstitium. Absence of WNT5a activity in the mutant lungs leads to increased expression of Fgf-10, Bmp4, Shh, and its receptor Ptc, raising the possibility that WNT5a, FGF-10, BMP4, and SHH signaling pathways are functionally interactive.