1. To assess the role of changes in lung collagen in pulmonary fibrosis, the content of this protein was measured in biopsy and autopsy lung from patients with cryptogenic fibrosing alveolitis (CFA), a fibrotic lung disorder of unknown cause. 2. The collagen concentration was measured in lung samples from 21 patients with CFA (14 autopsy and seven open-lung biopsy) and 17 normal subjects; total lung collagen was determined in the right lung of 10 patients who died from CFA and the results were compared with those from 10 normal lungs. 3. There was a wide variation in the collagen concentrations but the mean value (± sem ) for patients with CFA (217±13mg/g dry weight) was significantly higher ( P < 0.02) than that of the controls (155 ± 15mg/g dry weight). The mean collagen concentration of the autopsy samples (243 ± 20 mg/g dry weight) was significantly higher ( P < 0.05) than that of the biopsy samples (165 ± 24 mg/g dry weight). 4. The mean total collagen was markedly raised ( P < 0.001) in right lungs of patients with CFA (32.5 ± 4.3g) compared with normal lungs (14.0 ± 1.1g). When corrected for the predicted lung volume this difference in total lung collagen remained statistically significant ( P < 0.01, mean for patients 4.7 ± 0.7 mg/ml, controls 2.3 ± 0.2 mg/ml). 5. These results demonstrate an increased deposition of lung collagen in this form of pulmonary fibrosis. They also suggest that there is a greater collagen concentration in lungs of patients with later disease, indicating a progressive deposition of collagen during the course of the disease.