ABSTRACT.Increased synthesis of type I collagen, leading to increased ratios of type I to type III collagen in the lungs, has been observed in the lungs of animals with experimental pulmonary fibrosis. Similar changes in collagen type ratios have been observed in lungs of humans dying of idiopathic pulmonary fibrosis and of adult respiratory distress syndrome. In this study, lung collagen type ratios were examined in infants with acute and chronic lung disease. Tissue from the right lower lobes of neonates was obtained post mortem. Specific collagen types were quantitated by solubilization of lung collagen with CNBr and fractionation of the resulting mixture of peptides by column chromatography and polyacrylamide gel electrophoresis. Ratios of type I/III collagen were calculated for each lung sample using two independent pairs of marker peptides for these determinations. In some cases the ratio of type Y to type III collagen in these same lung samples was also quantitated. We observed a significant increase in the ratio of type I/III collagen in infants with a premortem diagnosis of chronic lung disease, usually preceded by respiratory distress syndrome. We also observed two infants with large changes in collagen type ratios who might have had pulmonary fibroplasia secondary to intrauterine lung disease. These data suggest that there may be several subsets of infants with respiratory distress syndrome, each having a different prognosis.