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OBJECTIVE: Polymyositis and dermatomyositis (PM/DM) are often complicated by interstitial pneumonitis (IP), which is an important cause of death. It has been reported that blood concentration of transforming growth factor-beta (TGF-beta), which is produced by a wide range of cells including endothelial cells and enhances the fibrotic changes in various tissues, is increased in PM/DM with IP. Endothelial damage is likely to exist in PM/DM. We studied the relationship between endothelial damage and IP in PM/DM. METHODS: Blood levels of sialylated carbohydrate antigen KL-6, TGF-beta, endothelin-1 (ET-1), thrombomodulin (TM), and plasminogen activator inhibitor-1 (PAI-1) were determined in 43 patients with PM or DM with or without IP, and the relationship between these measures was analyzed. RESULTS: Blood levels of KL-6 and TGF-beta were higher in the patients with IP than those without, and these measures were well correlated with each other. Levels of ET-1, TM, and PAI-1, all known to reflect the extent of endothelial damage, were also increased in patients with IP, and these measures correlated well with TGF-beta. CONCLUSION: Our data suggest that endothelial damage might play an important role through the production of fibrosis-enhancing factors such as TGF-beta or ET-1 in PM/DM.