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Lung autopsies from four neonates with Ebstein's anomaly or tricuspid valve dysplasia and gross cardiomegaly were examined. The mean cardiothoracic ratio was 92 +/- 0.5%. The degrees of pulmonary hypoplasia and pulmonary artery muscularization were assessed and were compared with those in lungs from four controls and four patients with diaphragmatic hernias. Lung and body weight ratios and radial alveolar counts, which reflect pulmonary hypoplasia and immaturity, were significantly decreased only in patients with diaphragmatic hernia. The thickness of the medical muscle layer in small pulmonary arteries was greater in patients with diaphragmatic hernia; however, in patients with tricuspid valve disease, it was relatively small. Abnormal vascular muscle extension was seen only in patients with diaphragmatic hernia. In the cases of tricuspid valve disease, although cardiomegaly and lung compression were severe, lung hypoplasia and immaturity were not, and neither abnormal medial thickening nor extension were found. Our results suggest that, at least in full-term infants with tricuspid valve disease, surgical relief of lung compression may improve respiratory function, even if the cardiomegaly is severe.