The role of changes in the relative amounts of types I and III collagen in cryptogenic fibrosing alveolitis was investigated using a biochemical method adapted for small amounts of human lung tissue. Measurements were performed in 18 biopsy and seven post-mortem lung samples from patients with cryptogenic fibrosing alveolitis and the results were compared with clinical, physiological, radiological and histological data. The mean percentage type III collagen, compared with type I collagen, was significantly lower in post-mortem lung samples from patients with cryptogenic fibrosing alveolitis who died from the disease compared with the biopsy lung samples (p less than 0.01) and with the control lung samples (p less than 0.05). There was a positive correlation between the percentage of type III collagen and response to treatment at six months as shown by the improvement in the percentage predicted Kco (p less than 0.005) and FVC (p less than 0.01) and with the radiological changes (p less than 0.05). These changes remained significant at one year (p less than 0.05 in all cases). These results suggest that patients with a higher proportion of type III collagen have earlier disease and show a better response to treatment than those with a lower proportion of type III collagen. The potential advantages of measuring collagen types for staging cryptogenic fibrosing alveolitis are discussed.