Pulmonary fibrosis is an irreversible accumulation of connective tissue in the interstitium of the lung. The pathogenesis of pulmonary fibrosis is not well understood. Research on animal models and studies of human lung disease suggest the initiating events may be a combination of pulmonary injury and the recruitment of inflammatory cells, mainly macrophages. A number of well characterized cytokines, including TGF-beta, have been either found in the injured lung or produced by inflammatory cells removed from the lung. In an animal model of pulmonary fibrosis, TGF-beta production is increased prior to collagen synthesis and is mainly produced by alveolar macrophages. In advanced idiopathic pulmonary fibrosis, a human fibrotic lung disease, extensive TGF-beta deposition can be detected by immunohistochemical staining, primarily in epithelial cells in areas of lung regeneration and remodelling. This suggests that the pathogenesis of the progressive fibrosis characteristic of this lung disease may be an aberrant repair process.