Purpose. Oxidative mechanisms play a major role in the aetiology and pathogenesis of cataract, especially in age-related cataract. Our study aims to investigate systemic oxidant and antioxidant markers in cataract patients. Methods. The activity of erythrocyte catalase and the level of malondialdehyde in plasma were measured in 40 patients with cataract and 60 healthy control subjects. The malondialdehyde level, as an index of lipid peroxidation, was determined by thiobarbitüric acid reaction according to Yagi. The determination of catalase activity was measured by a method that was defined by Beutler. Catalase enzyme activity and malondialdehyde level were evaluated to find out whether there was a significant difference in these variables. Analysis of variance was used by forming a general linear model that takes age and gender as the covariate. Results. CAT activity was found to be 13 920.2±847.9 U/l in cataract patients and 16 061.3±1126.6 U/l in control subjects. CAT activity in cataract patients was significantly lower than the control subjects (P=0.008). Plasma MDA level is significantly higher in patients with cataract 4.47 ± 0.35 nmol/ml compared to the control subjects 2.94±0.26 nmol/ml (P=0.0001). There was no significant difference between different cataract subgroups when erythrocyte CAT activities and plasma MDA levels were compared (P=0.322, 0.062). Conclusion. This study shows that oxidant/antioxidant balances alter in the presence of cataract. © 2004 Nature Publishing Group All rights reserved.