The gliding surface of the A1 pulley was studied in 20 cases of primary trigger finger by scanning and transmission electron microscope. In 12 normal specimens, the whole deep surface was covered uniformly by an amorphous extracellular matrix. In the pathological samples, there was the same general surface appearance but, also, areas, varying in shape and dimension where loss of the extracellular matrix had exposed the collagen fibres and a few cells of the middle layer of the pulley. There were also changes typical of "chondroid-metaplasia". These data were confirmed by transmission electron microscopy. The fragmented areas are probably the result of altered forces of friction between the pulley and the flexor tendon and may be the "gate" through which the forces of friction cause chondroid-metaplasia in the underlying fibrous tissue, a phenomenon recognised to be one step in the pathogenesis of trigger finger.