Chronic ulcerative colitis may be accompanied by a variety of epithelial changes, including loss of goblet cells, Paneth cell metaplasia, villous metaplasia, and dysplasia. Total colitis is also accompanied by an increased incidence of adenocarcinoma. All these changes are assumed to be secondary to repeated mucosal damage, but how they develop is unknown. Little attention has been paid to the enteroendocrine cell population, despite the postulated role of these cells as producers of trophic hormones. We describe two patients with long-standing ulcerative colitis who developed both adenocarcinoma and carcinoid tumours. In both, there were increased numbers of enteroendocrine cells in the uninvolved colonic mucosa. We suggest that an increased enteroendocrine cell mass may be part of a non-specific reaction to chronic mucosal injury, and by producing an elevated level of trophic hormones may act as a promoter in the development of neoplasia.