A well-differentiated colonic adenocarcinoma containing large numbers of gastrointestinal neuroendocrine cells is presented. The presence of neurosecretory granules was confirmed by electron microscopy. Immunocytochemistry showed large numbers of serotonin-containing tumor cells and lesser numbers of somatostatin, gastrin, motilin, secretin and neurotensin-containing cells. Some of these hormones are not normally present in the colon in significant numbers of cells. The presence of several cell types within a single tumor supports the concept that the normal epithelial cells of the gastrointestinal mucosa are derived from a common endodermal stem cell. There exists a spectrum of tumors ranging from the classical adenocarcinoma to the classical carcinoid, and this report identifies the position of this case within that spectrum.