Plasma myeloperoxidase (MPO) and glutathione peroxidase (GSH-Px) activities, and malondialdehyde (MDA), ceruloplasmin (Cp), transferrin (TF), thiol (SH), selenium (Se) and copper (Cu) levels were determined in patients with Behçet's disease (BD), in order to investigate whether the plasma antioxidant defense system is impaired in BD. When compared to controls, plasma MDA, Cp, Cu levels and also plasma MPO activity were significantly higher in patients, whereas plasma TF, SH and Se levels, and also plasma GSH-Px activity were lower in BD patients than those in controls. In addition, there were significant and positive correlations between MDA-Cp, MDA-Cu, MDA-MPO, MPO-Cp, GSH-Px-Se, Cp-Cu, and TF-SH parameters, but negative correlations between MPO-TF and Cp-TF parameters in BD patients. Based on these findings, it is concluded that plasma antioxidant defense system is insufficient and impaired in BD.