Colon carcinogenesis is a multistep process where oxygen radicals were found to enhance carcinogenesis at all stages: initiation, promotion, and progression. Since insufficient capacity of protective antioxidant system can result in cancer, the aim of this study was to examine the activity of antioxidant enzymes (superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase) and the levels of reduced glutathione, vitamin C, and vitamin E. The lipid peroxidation products were also determined by measuring malondialdehyde and 4-hydroxynonenal levels in colorectal cancer tissue collected from 55 patients. In these cases the activity of superoxide dismutase, glutathione peroxidase, and glutathione reductase was significantly increased while the activity of catalase was significantly decreased in cancer tissue. However, the level of nonenzymatic antioxidant parameters (glutathione, vitamin C, and vitamin E) was significantly decreased in cancer tissue. Further lipid peroxidation was enhanced during cancer development, manifested by a significant increase in malondialdehyde and 4-hydroxynonenal levels. The obtained results indicate significant changes in antioxidant capacity of colorectal cancer tissues, which lead to enhanced action of oxygen radicals, resulting in lipid peroxidation.