Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the increased production of autoantibodies and by systemic clinical manifestations and damage to multiple organs. The aim of the present study was to analyse matrix metalloproteinase (MMP)-9 activity in sera of patients with active and inactive SLE in order to evaluate its role in the pathogenesis and course of the disease, as well as its diagnostic value. We measured activity levels of MMP-9 and MMP-2, using both gel zymography and activity assay kits, in sera of 40 SLE patients and of 25 healthy controls. We found that MMP-9 activity, but not MMP-2 activity, is significantly elevated in the sera of SLE patients compared with sera samples of healthy controls. High activity levels of MMP-9 were determined in sera of 68% of the SLE patients. Elevated levels of MMP-9 were correlated with the presence of discoid rash, Raynaud phenomenon, pneumonitis, mucosal ulcers and anti-phospholipid antibodies. Changes in activity levels of MMP-9, but not of MMP-2, were observed in sera of the same patient at different periods of the disease course. High levels of MMP-9 did not correlate with disease activity index (SLEDAI, BILAG) in female patients, but correlated with SLE activity in the group of male patients. The results of the present study suggest that MMP-9 plays a role in the pathogenesis of SLE.