We have studied 15 cases of cryptogenic fibrosing alveolitis stained with a monoclonal antibody reactive with human tumour necrosis factor-alpha (TNF alpha), a cytokine that has been implicated in inflammation and fibrosis. Seven were examples of lone cryptogenic fibrosing alveolitis and eight were examples of cryptogenic fibrosing alveolitis in patients with systemic sclerosis. There was widespread staining of epithelial cells, particularly hyperplastic type II pneumocytes. Macrophages stained only weakly. In a control group of 10 patients dying of unrelated conditions, staining for TNF alpha was weak and, in the alveolar epithelium, was confined to a very occasional type II pneumocyte. The strong expression of TNF alpha in hyperplastic type II pneumocytes suggests that TNF alpha produced during alveolar epithelial regeneration may play a part in the fibrosis seen in this disease.