Aims: To measure the activity of oxygen free radicals and the level of antioxidant enzyme superoxide dismutase (SOD) in the synovial fluid (SF) of the temporomandibular joints (TMJs) of patients with temporomandibular disorders (TMD). Methods: Thirty-two patients were divided into 3 subgroups: anterior disc displacement with reduction, anterior disc displacement without reduction, and osteoarthrosis. Six healthy volunteers served as controls. A pumping procedure was used to take SF from the superior TMJ space. The concentration of lipid peroxidation products was assessed by means of the thiobarbituric acid reaction, and the level of SOD was assessed with spectrophotometry. Results: SF levels of lipid peroxides (LPO) in the control and patient groups were 0.010 (± 0.004) x 10-3 nmol/mg protein and 0.61 (± 0.121) x 10-3 nmol/mg protein, respectively. SF SOD levels were 4.61 (± 1.30) NU/mg protein and 9.83 (± 2.66) NU/mg protein, respectively. Both the concentration of LPO and the level of SOD activity were significantly higher in the TMD patients than in the normal control subjects (P .001 and P .01, respectively). There were no significant differences in the level of LPO or SOD between the 3 subgroups. Conclusion: These results demonstrate oxygen free radicals and antioxidant enzymes may be connected in the pathogenesis of TMD. Keywords: lipid peroxides, oxygen free radicals, superoxide dismutase, synovial fluid, temporomandibular disorders