Diffuse interstitial lung disease (DLD) is characterised by varying degrees of inflammation and fibrosis that result in derangement of the gas-exchanging units of the lung. A hallmark of these diseases is the abnormal deposition of collagen, which is a prime determinant of clinical course. This review considers the current information concerning collagen turnover in diffuse fibrotic lung disease. Considerable experimental evidence implicates both increased collagen production and reduced degradation in these diseases. Reduced degradation may result from both reduced production of collagenases and increased inhibition by tissue inhibitors. The known effects upon collagen turnover of novel therapeutic agents for pulmonary fibrosis (pirfenidone and interferon gamma) are discussed.