This study was performed to investigate the antioxidant mechanisms of polymorphonuclear leucocytes (PMN) in active stage of Behçet's Disease. PMN activities of myeloperoxidase (p < 0.02), superoxide dismutase (p < 0.001), catalase (p < 0.005), and glutathione peroxidase (p < 0.005) were significantly lower in the patients: the NADPH oxidase activity was significantly higher (p < 0.001) than those in controls. The plasma levels of ceruloplasmin (Cp), fibrinogen, and copper (Cu) were also significantly higher in the patients group (p < 0.001). Significant and positive correlations were found between the glutathione peroxidase and catalase activities (p < 0.001) and also between the plasma Cp and Cu levels (p < 0.001) in the patients group. However, no correlation was observed among the other enzyme activities. In the control group, a significantly positive correlation was present only between the plasma ceruloplasmin and Cu levels (p < 0.001). It was concluded that (impaired PMN functions) decreased enzyme activities in the antioxidant system and increased levels of oxygen free radicals may play a role in tissue damage in Behçet's disease.