An unusual case of multicentric ileal carcinoids and appendiceal endocrine carcinoma in association with Meckel's diverticulum was studied, with special attention given to the histogenesis of these neoplasms. A total of six ileal carcinoids, the largest of which was located in the wall of Meckel's diverticulum, were macroscopically and microscopically confirmed to be multicentric and revealed no visceral metastases. The histochemical and immunohistochemical profiles of the ileal carcinoids and the appendiceal carcinoma differed considerably: the former resembled subepithelial neuroendocrine cells and the latter resembled epithelial nonmucous cells and Paneth cells. The appendiceal carcinoma exhibited signs of endocrine differentiation, expressing somatostatin and vasoactive intestinal polypeptide, and secreted mucus. The tumor had metastasized to various organs. The carcinoids exhibited signs of neuroendocrine and glandular differentiation, expressing neuron-specific enolase, serotonin, chromogranin A, and endocrine granule constituent, and secreted little mucus. The data suggest different tumor cell origins or different grades of differentiation of the two types of intestinal endocrine cell tumor.