The role of NF-κB activation in tumor initiation has not been thoroughly investigated. We generated Ikkβ(EE)^IEC transgenic mice expressing constitutively active IκB kinase β (IKKβ) in intestinal epithelial cells (IECs). Despite absence of destructive colonic inflammation, Ikkβ(EE)^IEC mice developed intestinal tumors after a long latency. However, when crossed to mice with IEC-specific allelic deletion of the adenomatous polyposis coli (Apc) tumor suppressor locus, Ikkβ(EE)^IEC mice exhibited more β-catenin⁺ early lesions and visible small intestinal and colonic tumors relative to Apc^+/ΔIEC mice, and their survival was severely compromised. IEC of Ikkβ(EE)^IEC mice expressed high amounts of inducible nitric oxide synthase (iNOS) and elevated DNA damage markers and contained more oxidative DNA lesions. Treatment of Ikkβ(EE)^IEC/Apc^+/ΔIEC mice with an iNOS inhibitor decreased DNA damage markers and reduced early β-catenin⁺ lesions and tumor load. The results suggest that persistent NF-κB activation in IEC may accelerate loss of heterozygocity by enhancing nitrosative DNA damage.