In spite of unknown etiology, it is now accepted that reactive oxygen species (ROS) produced by neutrophils may be related to the pathogenesis of Behcet's Disease (BD). The objective was to investigate whether increased production of ROS may affect erythrocyte oxidant/antioxidant system in patients with BD. The levels of malondialdehyde (MDA), one of the end products of lipid peroxidation, in plasma and erythrocyte, and the activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), antioxidant enzymes, in erythrocyte, also C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) were measured in 22 patients in active stage of the disease and also in 30 healthy controls. Increased CRP, ESR, and MDA levels in plasma and erythrocyte and. increased SOD but decreased GSH-Px activities in erythrocytes were observed in the patients, when compared to the controls. In addition, significantly positive correlations between plasma and erythrocyte MDA levels, and erythrocyte MDA-CRP, MDA-ESR, MDA-SOD, SOD-ESR and SOD-CRP levels, but negative correlation between plasma MDA and erythrocyte GSH-Px, were found in BD patients. It may be suggested that increased production of ROS in BD, as reflected by higher plasma and erythrocyte MDA levels, may impair erythrocyte membrane integrity and also may lead to the alterations in the erythrocyte antioxidant defense system, as reflected by higher SOD and lower GSH-Px activities in erythrocytes. - Behget's disease; lipid peroxidation; glutathione peroxidase; superoxide dismutase; acute phase reactant (C) 2002 Tohoku University Medical Press.