The causative mechanism of tendon calcification ('calcifying tendinitis') is unknown. In this report, pathological human tendon samples were examined to give morphological and ultrastructural detail of the calcified regions and these findings were compared with those from normal tendon. Selected specimens were cryosectioned to enable histochemical and immunohistochemical comparison of the occurrence and distribution of specific matrix molecules in diseased and normal tendon tissues. The lack of collagen type II and alkaline phosphatase in the pathological regions suggests that the calcification process is not mediated through an endochondral transition. In contrast, the pathological areas were characterised by widespread labelling for chondroitin-4-sulphate/dermatan sulphate and intense pericellular localisation of chondroitin-6-sulphate.