The clinicopathologic significance of mucus production by adenocarcinoma of the colon and rectum was analyzed in retrospective study with stage matched non-mucus producing control carcinomas. Mucinous carcinoma of the colon and rectum comprised 132 (15%) of 893 cases of colorectal carcinoma. The rectum was the most common site (33% of cases). While 120 mucinous cancers had a poorer five-year survival than non-mucinous tumors (34% vs. 53%, p less than .005), these had a particularly bad prognosis in the rectum (18% 5 year survival vs. 49% for the non-mucinous tumor controls, p less than .00k). The theoretical basis for this location-dependent behavior is considered. From this study, distinctive clinico-pathologic features emerge. There were seven documented cases of ulcerative colitis and 8 additional patients gave a history of "colitis". An additional five patients had received prior pelvic irradiation. Of particular note was the fact that 31% of mucinous carcinomas were associated with villous adenomas, implying a histogenetic relationship. Moreover, this finding again emphasizes the neoplastic potential of the villous adenoma, especially in the rectum where the development of mucinous carcinoma is particularly ominous.