In the absence of active pulmonary disease, mycobacterial infection frequently causes arthritis and can be considered to initiate autoimmune diseases such as rheumatoid arthritis. Temporomandibular disorder (TMD) is a disease in which pain and impaired mandibular movement appear to arise directly from degenerative or inflammatory changes within the temporomandibular joint, but its precise pathogeny has not been elucidated. Here we examined whether mycobacterial infection is related to the pathology of TMD. The antibody levels against mycobacterial antigen in the synovial fluid (SF) of patients with TMD were assessed by enzyme-linked immunosorbent assay (ELISA). Six of 17 TMD patients (35%) were found to possess mycobacterial antigen-specific immunoglobulin (Ig) G but not IgM, while the six healthy volunteers possessed neither. Western-blot analysis was used to isolate the reacted antigen, and the IgG reacted strongly to 44-kDa antigen. The first 14 N-terminal amino acid sequences were determined, and computer analysis revealed that it was homologous to translational elongation factor Tu (EF-Tu) of Mycobacterium tuberculosis, which was a major target antigen for these antibodies. The 44-kDa protein of Mycobacterium bovis BCG (BCG) was identical with the EF-Tu of M. tuberculosis. We cloned the gene encoding the EF-Tu of BCG by using the synthesized oligonucleotide primers by means of polymerase chain-reaction. The gene was expressed in Escherichia coli. The protein was purified, and the antibody levels against this recombinant protein in the SF of TMD patients were assessed by ELISA. Our findings suggest that some cases of TMD are concerned with the synovial IgG against the EF-Tu of M. tuberculosis, and that the existence of the antibody is a clinical indication of TMD.