Fibronectin is a large fibril-forming extracellular glycoprotein that seems to be involved in joint diseases. The objective of this study was to investigate the expression of fibronectin in human temporomandibular joint disks obtained from patients with internal derangement and varying extents of disk tissue degeneration/regeneration with that of temporomandibular joint disks free of significant morphological alterations by means of immunohistochemical methods. Twelve adult human temporomandibular joint disks (10 diseased disks and 2 normal disks) were used in this study. Temporomandibular joint disks were fixed in 10% buffered formalin. Sections were then immunohistochemically processed using a monoclonal antibody specific to human fibronectin and streptavidin-biotin detection methods. Positive reactions to fibronectin were found in normal and diseased disk tissues but to a different extent. Normal disk tissues revealed weak fibronectin expression, which was mainly located along the collagen bundles. Temporomandibular joint disks with internal derangement exhibited a higher immunoreactivity. Distinct reticular fibronectin structures were found inside the diseased disk, particularly nearby the newly formed blood vessels, tears, and clefts. In the covering layer of the disk surface, fibronectin was expressed in a fascicular pattern running parallel to the disk surface. The findings suggest that temporomandibular joint disk tissue can express fibronectin and that the expression is more pronounced in disk specimens of patients with internal derangement of the temporomandibular joint, supporting a role of this glycoprotein in the degeneration/regeneration processes of human temporomandibular joint disk tissue.