The oxidative stress is considered to be involved in the pathophysiology of cancers. In the current study we explored the oxidative stress in patients with different cancers and corresponding benign diseases by evaluation of the level of lipid peroxidation products (MDA level) in the plasma and the activity of erythrocyte antioxidant defense enzymes superoxide dismutase (SOD) and catalase (CAT). Significantly higher plasma levels of lipid peroxidation products were detected in patients with early and advanced cancers in comparison to the healthy volunteers (mean 3.1 micromol/l and 2.3 micromol/l, p = 0.0003 and p = 0.029, respectively, t-test). In addition, 10-20 days after radical operations of cancer patients with normal postoperative recovery period, the plasma levels of MDA decreased and reached values close to the controls (mean 2.0 micromol/l). SOD in erythrocytes of patients with benign diseases and malignant solid tumors before and after surgery did not differ from that of the controls. In contrast, CAT activity of patients with early cancers was found to be significant higher than that of the controls (mean 22157.2 U/gHb vs. 12832.0 U/gHb, p = 0.032, t-test). A decrease of CAT activity was observed after surgery (mean 15225.0 U/gHb). In conclusion, our results suggest the presence of an increased oxidative stress accompanied by a lack of changes of erythrocyte SOD activity and an adaptive increase of CAT activity.