Many of the clinicopathologic features of neonatal respiratory distress syndrome (RDS) may be related to the inflammatory response mounted by the affected infant, although little is known about the interstitial component of this response. We have thus studied the local inflammatory response in this condition by immunohistochemical analysis of whole lung lobes, obtained at postmortem from 40 infants who died from acute RDS in the first week of life. All had demonstrated classical clinical history and histologic features. An archival subgroup from the early 1970s had never received ventilatory support. Immunohistochemical analysis demonstrated rapid temporal increase from birth in the mucosal density of CD68+ macrophages, MAC-387+ monocytes/macrophages, polymorphonuclear neutrophils, and tumor necrosis factor-alpha-immunoreactive cells, maximal in those dying at or after 72 h. Using a cationic probe specific for sulfated glycosaminoglycans (GAGs), the inflammatory infiltration was seen to be associated with striking loss of endothelial, basement membrane, and interstitial GAGs, which was almost complete by 48-72 h. GAG degradation products were found within hyaline membranes in all infants dying after 48 h. This study confirms that neonatal RDS is characterized by intense interstitial inflammation, significantly underestimated on routine staining. This begins within hours of birth and is maximal by 72 h of age. Breakdown of sulfated GAGs within the extracellular matrix follows the same time course and may explain much of the physiologic derangement characteristic of this condition.