Eighty-nine A1 pulleys from 65 patients with trigger digits and 20 control A1 pulleys from fresh-frozen cadavers were studied comparatively with histology, immunohistochemistry, and transmission electron microscopy. In both normal and pathologic specimens, the A1 pulley was composed of two layers: an outer, vascularized, convex layer and an inner, concave, friction, flexor tendon gliding layer. In the latter, the cells and adjacent matrix had several characteristics of fibrocartilage, including chondrocytes. In trigger digits, the number of chondrocytes and adjacent extracellular matrix was significantly increased when compared with controls. There was no evidence of a synovial cell layer on the surface of the A1 pulleys in either normal or trigger digits. We conclude that the underlying pathobiological mechanism for triggering at the A1 pulley is characterized by a fibrocartilage metaplasia.