Colorectal cancer is a major public health concern particularly in developed countries. Despite decades of advances in the treatment and prevention of colorectal cancer, it remains the second most common cause of cancer death. There now exists convincing evidence that reactive oxygen species play an important role in the etiology and progression of a number of human diseases including colorectal cancer. Reactive oxygen species may damage all types of biological molecules. However, proteins are possibly the most immediate vehicle for inflicting oxidative damage on cells since they are often catalysts rather than stoichiometric mediators, hence, the effect of damage to one molecule is greater than stoi-chiometric.The aim of the study was to investigate oxidative protein damage in patients with colorectal cancer and its correlation with the clinical stage of the disease.Material and methods. The study group comprised 102 patients operated on for colorectal cancer in different clinical stages of the disease. Plasma carbonyl levels were determined using Levin's method.Results. Patients in all tumor groups showed significantly higher levels of plasma carbonyls when compared to healthy people. We observed an increase in mean plasma carbonyl levels correlating with an increase in the degree of disease advancement.Conclusions. This study demonstrates that reactive oxygen species may have a role in pathogenesis of colorectal cancer. The outcomes of this research seem to confirm that antioxidants may play a role in chemoprevention of colorectal cancer.