The incidence of small subpleural lung cysts was analyzed for autopsies of 98 patients with Down syndrome (DS): 9 fetuses or stillborns and 89 liveborns. Such cysts were not seen in any of the fetuses or stillborn infants; they were identified in 18 (20%) liveborn DS patients but were present in only 1 of 11 DS patients who survived less than 4 weeks. Such cysts were not found in lungs of 61 non-Down patients with ostium atrioventricular commune (AVC), the most frequent congenital cardiac lesion of DS, or in those of 60 non-DS patients with bronchopulmonary dysplasia. Although subpleural lung cysts were more frequent in DS patients with than in those without congenital heart disease, the difference in incidence of such cysts between 25 liveborn DS patients with AVC and 61 non-DS patients with AVC was highly significant (chi 2 - 19.46, P less than .001). The data suggest that the lung cysts in DS are an intrinsic, albeit not invariant, feature of the disease and not necessarily secondary to congenital heart disease or sequelae thereof. We suggest that they may result from reduced postnatal production of peripheral small air passages and alveoli, which in turn may reflect the slow rate of cell proliferation that appears to be a general feature of DS.