Tumour necrosis factor alpha (TNF-alpha) transgenic mice, which overexpress TNF-alpha in the lungs, develop interstitial pneumonitis resembling idiopathic pulmonary fibrosis (IPF) in humans. Transgenic mice were used to study molecular pathogenesis of interstitial pneumonitis with regard to sequential histological changes and cytokine network induced by TNF-alpha. The authors divided the histological process of interstitial pneumonitis into three stages: early stage with lymphocytic infiltration in alveolar septa, middle stage with recruitment of macrophages, and late stage with hyperplasia of epithelial cells and mild fibrosis. As for cytokine network, prolonged overexpression of TNF-alpha along with increasing interleukin 6 (IL-6) were associated with the progression of interstitial pneumonitis. Increasing IL-1 was found only in the early stage, the beginning of lymphocyte proliferation. The mRNA level of an anti-inflammatory cytokine, IL-10, was constantly enhanced in the lungs of transgenic mice. However, transforming growth factor beta 1 (TGF-beta 1) protein decreased, which is closely associated with prolonged TNF-alpha synthesis, resulting in development of chronic inflammation and less severe fibrosis in the lungs of this animal model, analogous to inflammatory stage of human IPF. TNF-alpha transgenic mice enabled the analysis of the sequential process of interstitial pneumonitis as a model of IPF pathogenesis in humans, the results of which will give rise to new therapeutic measures for human IPF.