OBJECTIVE The aim of this study was to determine the possible association of oxidant/antioxidant status and recurrent aphthous stomatitis (RAS). METHODS The study consisted of thirty-one patients with RAS and thirty-two healthy controls from whom saliva and blood samples were collected. Superoxide dismutase (SOD), glutathione peroxidase (GSHPx) and catalase (CAT) were measured in erythrocytes and total antioxidant status (TAS) was measured in plasma and saliva. RESULTS Erythrocyte SOD activity was significantly lower in RAS patients in comparison to healthy controls (P=0.012). No significant differences were found in erythrocyte GSHPx, CAT activities, and salivary and plasma TAS between RAS patients and control subjects (p>0.1). CONCLUSIONS Changes in SOD activity may be important in the inflammatory reactions observed in RAS, but other tested defense systems such as CAT and GSHPx do not seem to play a primary role in the aetiopathogenesis of RAS. Moreover, the antioxidant system in saliva and plasma is not as affected as in erythrocytes in RAS patients, and therefore it may not be considered an appropriate indicator of the body's total antioxidant status.