To assess the biological significance of human chorionic gonadotropin (HCG) detection in large bowel carcinomas, we have studied immunohistochemically 50 colorectal carcinomas, 20 adenomas, 8 ulcerative colitis, and 10 normal colonic mucosae. The HCG-immunoreactive cells were found in 26 carcinomas (52%). Positivity was not detected in any normal mucosa or benign lesions. Cells containing HCG predominated in mucinous (80%) and poorly differentiated carcinomas (92%). No trophoblastic differentiation could be demonstrated in any tumor. Human chorionic gonadotropin was detected more frequently in carcinomas invading the entire bowel wall (67%) than in those confined to the submucosa or muscularis propria (30%). Fifteen of 19 cases (79%) with lymph node and/or hepatic metastases had HCG in the primary tumor, whereas only 9 of 23 cases (32%) without metastases showed HCG immunoreactivity. The eight patients with hepatic metastases had HCG in the primary tumor. Thus, the immunohistochemical detection of HCG in colorectal carcinomas may be a biological marker of prognostic significance.