&NA;A rare case of mucinous cystadenocarcinoma arising in the sigmoid colon, found accidentally during an operation for cholecystolithiasis, is reported. The tumor was located 40 cm from the anal verge, and had two histologic variations consisting of a large distended cystic lesion and branching cystic channels with papillary proliferation of the epithelium. Serial sections revealed the existence of a luminal communication between the two lesions. The tumor tissue was seen mainly in the muscularis propria with no mucosal involvement. The papillary portion had a highly differentiated appearance, giving rise to considerable difficulty in determining whether it was benign or malignant. In a localized area, however, the tumor invaded into the subserosa and showed distinctive atypical changes. The tumor cells showed intense reactivity for carcinoembryonic antigen. This mucinous cystadenocarcinoma was considered to be originated from an enterogenous cyst, a possible derivative of duplication of the colon. The differential diagnosis concerning this rare tumor is also discussed.