Colon cancer is one of the most serious complications of inflammatory bowel diseases, especially ulcerative colitis (UC). Previous studies have shown that characteristic immunological event during inflammation in UC is the expression of T helper-type 2 (Th2) cell-derived cytokines. In this study, we investigated the influence of a predominant Th2-type cytokine response in colitis on carcinogen-induced colon tumors. Wild type (WT), interferon gamma (IFN-γ) gene deficient (−/−) [Th2 dominant] or interleukin (IL)-4−/− [Th1-dominant] mice of BALB/c background were used in this study. To compare tumor formation, mice were given the carcinogen azoxymethane (AOM) and intrarectal administration of trinitrobenzene sulfonic acid (TNBS), to induce colitis. Thirty-three weeks after initial treatment, the total colon was examined. When IFN-γ−/− mice were treated with AOM and TNBS, significantly higher number of tumors were seen (8.4 ± 1.7) than in WT (3.3 ± 2.9) or IL-4−/− (3.1 ± 3.4) mice, which received identical treatments. A separate set of experiment, using less doses of AOM and TNBS also showed the higher frequency of tumor formation in IFN-γ−/− mice than in IL-4−/− mice. Histologically, the tumors were well- or moderately-differentiated adenocarcinomas. No invasion into the submucosal or serosal layers of the intestine was seen. In immunohistological staining, some tumors in IFN-γ−/− mice showed distinct nuclear expression of β-catenin, in contrast to the strong membrane staining seen in tumors of IL-4−/− mice. In conclusion, colonic inflammation associated with Th2-donimant cytokine responses enhanced the formation of malignant neoplasms.