A previous study of hyaline membrane disease of the newborn demonstrated that the hyaline membrane was derived from necrotic epithelial cells, and it was suggested that a no flow-reflow episode affecting the pulmonary circulation might account for the cell necrosis. An unexpected statistical finding in that study was a highly significant negative correlation between hyaline membrane disease and chorioamnionitis. Because it seemed to us that fetal infection could be expected to produce the circulatory pathophysiology suggested to produce hyaline membrane disease, we reexamined the issue by reviewing a group of autopsied infants. Data were collected on clinicopathologic variables from 42 stillborn and 54 liveborn infants less than or equal to 12 h of age with lung and placenta slides available for review. Correlations and multivariate regression analysis showed that the inflammatory responses in membranes, cord, and lung are interrelated but that hyaline membrane disease develops independently. However, lattice theory analysis, which represents the pathogenesis of a progressive disease process as edges along a mathematical lattice or hyperdimensional cube, showed that this separation was not absolute. Despite the negative correlation of hyaline membrane disease and fetal pneumonia, there were 10 (10%) patients with both conditions and a total of 19 (20%) with hyaline membrane disease and inflammation of membranes, cord, and/or lung. We suggest that the coexistence of hyaline membrane disease and fetal pneumonia in some patients implies a possible pathogenetic relationship between the two entities. Given the nature of the two processes, it seems likely that the direction of causality is that fetal pneumonia would give rise to the hyaline membrane disease of the newborn.