Behcet disease (BD) and recurrent aphthous stomatitis (RAS) are two distinct diseases of unknown aetiology which are both characterized by oral aphthae. The aim of this study was to determine the possible association of both diseases with antioxidant status and nitric oxide levels. Twenty-six patients (17 female, nine male) with RAS, 28 patients (17 female, 11 male) with BD and 31 (22 female, nine male) healthy control subjects were included in the study. Blood samples were studied for erythrocyte superoxide dismutase (SOD) and catalase (CAT) activities and plasma nitric oxide (NO) levels. Erythrocyte SOD activity in BD patients was significantly higher than in RAS patients and controls. Although SOD activity in RAS patients was higher than in controls, the difference was not statistically significant. No significant differences in CAT activities or NO levels were found between the three groups. In conclusion, changes in SOD activity may be important in the inflammatory reactions observed in BD and RAS, but NO does not seem to play a primary role in the aetiopathogenesis.