Chronic infection and inflammation considerably contribute to environmental carcinogenesis. Reactive oxygen and nitrogen species generated from inflammatory and epithelial cells may play an important role in inflammation-related carcinogenesis by causing DNA damage. We demonstrated that 8-nitroguanine, a potentially mutagenic nitrative DNA lesion, was formed at the sites of carcinogenesis in various clinical specimens and animal models. 8-Nitroguanine was formed in bile duct epithelium of animals and patients infected with the liver fluke Opisthorchis viverrini, which causes cholangiocarcinoma. 8-Nitroguanine formation was also observed in gastric gland epithelial cells of patients with Helicobacter pylori infection and in hepatocytes of patients with chronic hepatitis C. The formation of this DNA lesion in atypical cells of patients with cervical intraepithelial neoplasia, caused by human papilloma virus, was increased with its grade. 8-Nitroguanine formation in cancer cells of patients with nasopharyngeal carcinoma (NPC), associated with Epstein-Barr virus (EBV) infection, was significantly stronger than that in nasopharyngeal epithelium of EBV-mediated nasopharyngitis. Moreover, in patients with soft tissue tumor, strong 8-nitroguanine formation was closely associated with a poor prognosis. On the basis of these findings, we have proposed that 8-nitroguanine can be used as a biomarker to evaluate the risk of infection- and inflammation-related carcinogenesis and the prognosis of cancer patients. In this chapter, we discuss the significance of 8-nitroguanine in inflammation-related carcinogenesis and tumor development.