Oxidant/antioxidant balance has been suggested as an important factor for initiation and progression of cancer. In order to determine whether the degree of oxidative DNA damage and antioxidant enzyme activities in plasma obtained from patients with gastric and colon cancer who undergo resection can be used as a useful prognostic predictor, plasma level of 8-hydroxydeoxyguanosine (8-OHdG), activities of glutathione peroxidase (G-Px) and superoxide dismutase (SOD) were examined. 19 patients with gastric cancer and 26 patients with colon cancer who were undergoing resection of tumor were included by the study. Venous blood samples were taken just before the surgery. Plasma level of 8-OHdG was determined with ELISA, SOD and G-Px activities in plasma were measured by spectrophotometric kits. 8-OHdG level and activitiy of G-Px were found to be decreased, SOD activity was found to be increased in both gastric and colon cancer groups as compared to control group. Alpha fetoprotein was found to be correlated with G-Px in the gastric cancer group and correlated with 8-OHdG in the colon cancer group. SOD activity was correlated with CA-15-3 in the gastric cancer group. Low plasma level of 8-OHdG and altered antioxidant activity may implicate the deficient repair of oxidative DNA damage in patients with gastric and colon cancer. Those measured parameters were not found to be related with histopathological data but correlated with some tumor markers.