Significance of expression of cytokine induced nitric oxide synthase (iNOS) in colorectal carcinoma remains controversial. The aim of the current study was to elucidate the significance of immunohistochemical expression of iNOS in colorectal carcinoma. The subjects were the 54 patients with colorectal carcinoma, including 13 with obstructing carcinoma. The immunohistochemical expression of iNOS was examined and the clinicopathological data were compared between iNOS-positive and iNOS-negative carcinomas. Of 54 colorectal carcinomas, there were 27 tumors with iNOS expression. The proportion of tumors occurring in the rectum was significantly lower in iNOS-positive carcinomas than in iNOS-negative carcinomas (p=0.028). The incidence of lymph node metastasis and lymphatic invasion in colorectal carcinomas with iNOS expression (63.0%, 17 out of 27 and 96.3%, 26 out of 27, respectively) were significantly more frequent than those in carcinomas without iNOS expression (29.6%, 8 out of 27 and 70.4%, 19 out of 27, respectively) (p=0.013 and p=0.011, respectively). The proportion of obstructing carcinoma in tumors with iNOS expression (37.0%, 10 out of 27) was significantly higher than that in tumors without iNOS expression (11.1%, 3 out of 27; p=0.020). Our results could suggest that iNOS expression can be an indicator of malignant potential in colorectal carcinoma and that it may be one of the biological features of obstructing carcinoma of the colon and rectum. Moreover, our results demonstrated that nitric oxide might be associated with the difference in the mechanism of carcinogenesis in the colon and the rectum.