Colorectal cancer (CRC) is one of the most common human malignancies, affecting one of 20 persons in areas with high socio-economic standard. In Romania, the frequency of colorectal cancer is growing rapidly placing the country among countries with an average incidence of the disease. There are some etiologic factors involved and treatment of disease is carried out after proper staging. Biochemical mechanisms underlying malignant transformation in colorectal cancer are not all fully understood, therefore our work trying to enter in the path of oxygen metabolism at patients surgically treated. The aim of the study is to follow the production of active metabolites of oxygen, in the dynamics of the surgical procedure, and how the endogenous natural protection systems are activated, following the invasive procedure. Oxidative stress biochemistry assays, realized before and after surgical excision showed a direct relationship between the production of reactive oxygen species and the presence of tumor, without being able to distinguish exactly if malignant tissue is able to induce oxidative stress, or the latter occurs due to neoplastic changes. Based on the results we can say with certainty that the reactive oxygen species ROS primary attack occurs in the lipids, and then the proteins, following activation of endogenous antioxidant defence.