Previously unreported lung disease found at autopsy in 2 young infants with Down's syndrome and congenital heart disease (complete atrioventricular canal malformation with left-to-right shunt) is described. The perinatal and neonatal period was unremarkable, and there was no history of mechanical ventilation or administration of high concentration of oxygen for extended periods. In 1 of the cases respiratory symptoms and hyperinflation with focal cystic changes in the lung fields on chest X-ray were noted at 5-7 months of age. Pathologically there was cystic dilatation of alveoli with focal cuboidal metaplasia of alveolar epithelium and mild to moderate focal alveolar septal fibrosis. Wilson-Mikity syndrome, congenital pulmonary lymphagiectasia, bronchopulmonary dysplasia, and idiopathic interstitial fibrosis of lungs were ruled out on clinical and/or pathologic grounds. Factors such as compression of bronchi by enlarged pulmonary arteries or cardiac chambers, peribronchiolar accumulation of fluid, pulmonary hypoplasia occurring in Down's syndrome, and episodes of pulmonary arterial hypoperfusion associated with severe congenital heart disease may be related to the pathogenesis of the lesion.