Oxidative damage to macromolecules is thought to be an important etiologic factor in the development of several diseases including cancer. The aim of our study was to evaluate the protein and lipid oxidation in patients with gastrointestinal cancers and to investigate the relationship between protein and lipid oxidation, and gastrointestinal cancers. In our study, we included 108 gastrointestinal cancer patients and 35 healthy volunteers. Patients were divided into 3 groups: the 1st group included patients who had pancreas cancer, the 2nd group patients who had colorectal cancer,and the 3rd group patients who had liver and esophagus cancer. We investigated changes in serum protein carbonyl (PCO) and plasma nitrotyrosine (NT) levels, as an indicator of protein oxidation and peroxynitrite formation, malondialdehyde (MDA) and tumor necrosis factor alpha (TNF-alpha) levels in gastrointestinal cancer (GIC) patients and compare with healthy control groups. Malondialdehyde, a lipid peroxidation marker, was measured by the thiobarbituric acid method. PCO, TNF-alpha and NT levels were measured by using kits. The levels of MDA, PCO, TNF alpha and NT were significantly higher in patients with gastrointestinal cancer compared to those of control group (p<0.05). It is found that there is a significant positive correlation between MDA and PCO, and a negative correlation between PCO and NT levels in the patient group.