In order to compare histologic subtypes and endocrine profiles, immunohistochemical and silver stains were performed on 120 appendiceal carcinoids. Forty-three were predominantly insular; 21 were mixed insular, glandular, and trabecular; 33 were goblet cell; 17 were tubular; and five were clear cell carcinoids. Insular, mixed, and clear cell carcinoids were generally diffusely argentaffin and positive for chromogranin, neuron-specific enolase (NSE), and serotonin. Occasional tumors of insular or mixed patterns had scattered cells that stained weakly for glucagon, calcitonin, adrenocorticotrophic hormone (ACTH), somatostatin, cholecystokinin (CCK), human pancreatic polypeptide (HPP), or gastrin. Most had S-100-positive sustentacular cells. Less than half were positive for carcinoembryonic antigen (CEA). Many were cytokeratin-positive, but often focally. Goblet cell carcinoids contained few endocrine cells, but these were strongly argentaffin and positive for serotonin in nearly all, and positive for HPP in almost a third. Tubular carcinoids lacked argentaffinity and serotonin but were diffusely and strongly positive for glucagon. All goblet cell and tubular carcinoids were diffusely positive for CEA and cytokeratin. Somatostatin stained strongly in a single tumor, which had psammoma bodies and was in a patient with neurofibromatosis. In all groups, argentaffinity correlated with serotonin positivity, and argyrophilia with chromogranin positivity, although the latter was somewhat more sensitive. We conclude that among appendiceal carcinoids, the endocrine content varies according to histologic subtype.