Collagen in lung tissue was examined from patients with adult respiratory distress syndrome, from patients who did not have this disease but required mechanical ventilation and oxygen treatment, and from patients without overt lung disease. Cyanogen bromide peptide mapping techniques were used to determine the ratio of type I to type III collagen present in these lungs. In the fibrotic lungs from patients with adult respiratory distress syndrome a shift was found in the ratio of type I to type III from the normal value of 2:1 to a mean value of 3.4:1. In patients with normal lungs and those with other lung diseases collagen type ratios were normal. Our data suggest that (i) changes in lung collagen of patients with adult respiratory distress syndrome resemble those previously described in patients with idiopathic pulmonary fibrosis, although the changes occur much more rapidly in the former; (ii) the increased content of collagen in lungs of patients with adult respiratory distress syndrome shown by others is predominantly of type I collagen; and (iii) the stimulus to the lung to produce excess type I collagen relative to type III is not solely of iatrogenic origin--that is, resulting from oxygen or ventilator treatment.