Oxidative stress has a complex effect on cancer development. To further study this process, we induced colon tumors with azoxymethane (AOM) in mice deficient for uncoupling protein-2 (UCP2). UCP2 has recently emerged as a negative regulator of mitochondrial oxidant production. When overexpressed, UCP2 protects cells from oxidative stress, while its absence may cause abundance of reactive oxygen species, release of pro-inflammatory cytokines and persistent activation of nuclear factor kappaB (NF-κB), a pleiotropic transcription factor with an increasingly recognized role in cancer. Here we show that Ucp2−/− mice develop more aberrant crypt foci and colon tumors than Ucp2+/+ littermates when examined 24 weeks after the completion of treatment with AOM (10 mg/kg i.p. weekly for a total of 6 weeks, n=8–12). This effect is primarily seen in the proximal colon of Ucp2−/− mice (P < 0.05), in association with changes indicative of increased oxidative stress (increased staining for malondialdehyde and inducible nitric oxide synthase), enhanced NF-κB activation (increased levels of phosphorylated IκB and increased nuclear presence of p65) and a disrupted balance between intestinal epithelial cell proliferation (greater 5-bromo-2′-deoxy-uridine incorporation rates and increased phosphorylation of ERK1/2 and AKT) and apoptosis (decreased number of terminal deoxynucleotidyltransferase-mediated nick-end-labeling (TUNEL)-positive cells and increased expression of Bcl-2). In conclusion, our findings provide the first in vivo evidence for a link between UCP2 and tumorigenesis and indicate the need for additional studies to assess the role of mitochondrial uncoupling in cancer development.