Approximately 20% of human cancers are estimated to develop from chronic inflammation. Recently, the NF-κB pathway was shown to play an essential role in promoting inflammation-associated cancer, but the role of the JAK/STAT pathway, another important signaling pathway of proinflammatory cytokines, remains to be investigated. Suppressor of cytokine signaling-1 (SOCS1) acts as an important physiological regulator of cytokine responses, and silencing of the SOCS1 gene by DNA methylation has been found in several human cancers. Here, we demonstrated that SOCS1-deficient mice (SOCS1^−/−Tg mice), in which SOCS1 expression was restored in T and B cells on a SOCS1^−/− background, spontaneously developed colorectal carcinomas carrying nuclear β-catenin accumulation and p53 mutations at 6 months of age. However, interferon (IFN)γ^−/−SOCS1^−/− mice and SOCS1^−/−Tg mice treated with anti-IFNγ antibody did not develop such tumors. STAT3 and NF-κB activation was evident in SOCS1^−/−Tg mice, but these were not sufficient for tumor development because these are also activated in IFNγ^−/−SOCS1^−/− mice. However, colons of SOCS1^−/−Tg mice, but not IFNγ^−/−SOCS1^−/− mice, showed hyperactivation of STAT1, which resulted in the induction of carcinogenesis-related enzymes, cyclooxygenase-2 and inducible nitric oxide synthase. These data strongly suggest that SOCS1 is a unique antioncogene which prevents chronic inflammation-mediated carcinogenesis by regulation of the IFNγ/STAT1 pathways.