AIM:To discover the pathogenesis of concomitant exotropia by comparing the microstructure pathological changes and the expression of transforming growth factor (TGF-β1) of extraocular muscle of concomitant exotropia with normal humans. METHODS:Thirty pieces of extraocular muscle of concomitant exotropia patients were divided into three groups, each group included ten pieces of extraocular muscle, the fourth group included six pieces of extraocular muscle of normal humans Each group was observed by the light microscopy and Masson trichrome collagen staining, and TGF-β1 was determined with immunohistochemistry. RESULTS: Muscle tissue atrophy, muscle fiber size and number decreases,fibrosis, hyalinization and degeneration of muscle fibers were found in the weak side muscle of concomitant exotropia under microscope and a lot of blue collagen fibers were found between muscle bundles and the surrounding of muscle cells in the weak side muscle of concomitant exotropia after Masson trichrome collagen staining. All the extraocular muscle expressed TGF-β1,which showed buffy granules deposited in cytoplasm regularly.The expression of TGF-β1 in congenital exotropia group and constant exotropia group was higher than that in the normal human, the difference was statistically significant. CONCLUSION: In the pathological changing of extraocular muscle of concomitant exotropia, TGF-β1 plays a facilitating role.