Objectives: In a prospective design, we investigated xanthine oxidase (XO) activity, which is a marker of increased tissue DNA turn-over and catabolism of purine bases in the process of the tissue proliferation in nasal polyps, and lipid peroxidation, which occurs as a result of this enzymatic reaction, leading to the formation of free radical side products. Materials and Methods: The study included patients with nasal polyps (n=16), and those who underwent nasal surgery (concha resection and dacriocystorhinostomy) for diagnoses other than nasal polyps (n=10). Polyp tissues obtained by functional endoscopic sinus surgery and control specimens were homogenized for biochemical analyses. Xanthine oxidase, a marker of cell proliferation and DNA metabolism, and malondialdehyde (MDA), the end product of lipid peroxidation, were measured in tissue homogenates. Results: Significantly increased XO activity and MDA levels were found in polyp tissues compared to control mucosa specimens (p<0.05). Conclusion: Statistically significant increases in both parameters may strongly suggest metabolic changes including increased tissue DNA turn-over and increased lipid peroxidation in polyp tissues.