The aim of this investigation was to evaluate the immunohistochemical expression of cyclooxygenase-2 (COX-2) in the temporomandibular joint (TMJ) and to compare it with that control specimens. Expression of COX-2 in the TMJ disc and the synovial membrane in 26 human TMJ samples (internal derangement of TMJ; n=16, and control; n=10) was measured by an immunohistological technique using paraffin-embedded tissue and specific antihuman COX-2 polyclonal antibody. There were obvious distinction of COX-2 immunoreactivity between the control specimens and internal derangement cases, in the region of posterior and/or anterior loose connective tissues. In particular, intensive COX-2 expression was detected in the synovial membrane of internal derangement cases. The findings of the present study suggest that COX-2 might be an important mechanism regulating inflammation in the synovial membrane with internal derangement of TMJ.