A series of five benign and 60 malignant colonorectal neoplasms has been examined immunohistochemically for the presence of HCG. This hormone was not demonstrated in any of the benign tumours but was present in 43% of the malignant neoplasms. The incidence of HCG secretion was unrelated to the sex of the patient but tended to be decreased in patients of advanced age. The HCG-containing tumours, which were predominantly from the left side of the large intestine, had all penetrated the full thickness of the bowel wall while a significant proportion of those tumours lacking HCG were still confined to the bowel wall. Despite the greater degree of local aggressiveness shown by the HCG-secreting tumours there was no correlation between HCG production and the presence of local metastases but, as the presence of HCG is associated with local invasion, it is suggested that preoperative immunohistochemical studies of HCG in biopsies of large bowel neoplasms may be of value in the planning of surgical procedures.