OBJECTIVE: Osteoarthritis of the knee (KOA) is a common, age-related, joint disorder associated with loss of articular cartilage, osteophyte formation, sub-chodral bone change and synovitis. Recent studies have shown that reactive oxygen species (ROS) may participate in the initiation and progression of KOA. This study examines potential changes in the activities of antioxidant enzymes (superoxide dismutase, both isoenzymes zinc-copper superoxide dismutase and manganese superoxide dismutase) and glutathione transformation enzymes (glutathione peroxidase, glutathione reductase and glutathione-S-transferase) in synovial fluid of KOA patients, and estimates their relationship to the degree of lipid peroxidation in synovial fluid evaluated by malondialdehyde concentration, synovial fluid viscosity, type and duration of KOA. DESIGN: Synovial fluid samples obtained by transdermal arthrocentesis from 41 patients with KOA (23 had primary KOA and 18 had secondary KOA) and 22 control subjects were analyzed. Activities of antioxidant enzymes were analysed with the use of kinetic method, MDA concentration was measured fluorometrically by the Ohkawa method, and synovial fluid viscosity was measured using a cone-late viscometer Brookfield DV-II+ and a test by Ropes. RESULTS: Patients with KOA had significantly increased activities of all enzymes when compared to the control subjects for both KOA subgroups. The synovial fluid viscosity was significantly decreased and the synovial fluid test by Ropes was abnormal in KOA patients, mainly in the secondary KOA subgroup. The activities of all antioxidant enzymes were significantly negatively correlated with synovial fluid viscosity and duration of KOA. CONCLUSIONS: Patients with KOA display abnormal antioxidant status of synovial fluid with increased activities of antioxidant enzymes and decreased synovial fluid viscosity. Furthermore, synovial fluid viscosity, and activity of GR can be used to distinguish the primary from the secondary type of KOA.