To clarify the role of oxidative stress in aging of colorectal tissue, we analyzed the 8-hydroxydeoxyguanosine (8-OH-dG) levels in colorectal biopsy samples from normal tissue of patients with either colorectal cancer (n = 15) or benign colorectal polyps (n = 40). An age-associated increase of 8-OH-dG was observed (p =.002), although the 8-OH-dG levels were not significantly different between the patients with cancer and those with polyps. These results suggest an increased level of 8-OH-dG formation in human colorectal tissue with age.