Behcet's disease (BD) is a chronic inflammatory disorder of unknown aetiology, and recognised as a multi-system vasculitis. It has been postulated that an imbalance of the oxidant and antioxidant systems related to the disease are important in its pathogenesis. Previous publications have reported increased levels of enzymatic antioxidant defence systems in patients with BD. The non-enzymatic antioxidant systems, including vitamin C and uric acid, were looked for in the present study. For this aim, the serum malondialdehyde (MDA), an end product of lipid peroxidation, and vitamin C and uric acid, as endogenous antioxidants, were determined in 20 patients with BD (11 in active and 9 in inactive periods) and 20 healthy subjects. The MDA level was significantly higher in both the active and inactive period patients compared with the control group (p < 0.001, p < 0.05, respectively). The MDA level was also significantly higher in the active period patients compared with the inactive period patients (p < 0.05). The vitamin C levels were significantly lower in both the active and inactive period patients compared with the control group (p < 0.001, p < 0.05, respectively). There was no significant difference in the vitamin C level between the active and inactive period patients (p > 0.05). There was also no significant difference in uric acid levels between the groups (p > 0.05). In the patients group, a negative correlation was found between the levels of serum MDA and vitamin C (r=-0.517; p < 0.05). Our results indicate that decreased vitamin C and increased MDA levels reflect the increased levels of oxidative stress in BD patients, and this situation may be important in relation with its pathogenesis.