Appendiceal carcinoids with glandular differentiation pose difficulties in classification and prediction of clinical behavior. Sixty-four such cases were divided into three histologic groups on the basis of routine and immunohistochemical stains: (1) Tubular carcinoids were small and confined to the appendix, had small amounts of intraluminal mucin with few or no goblet cells, were nonargentaffin, lacked serotonin, and were diffusely positive for glucagon. All ten with follow-up (mean, 17 months) were without metastasis. (2) Goblet cell carcinoids were confined to the appendix and mesoappendix, circumferentially surrounded the appendiceal lumen, and were often not suspected grossly. Histologically, they were often mixed with small crypt-like glands and were serotonin positive. All 22 with follow-up (mean, 19 months) were without metastasis whether or not right hemicolectomy was performed. (3) Mixed carcinoid-adenocarcinomas showed spread into the cecum or adjacent viscera at the time of diagnosis and had a large carcinomatous pattern with areas of mucinous, signet-ring, or single-file structure, in addition to goblet cell or insular carcinoid. All patients had right hemicolectomies, and all but two with follow-up died of the disease (mean, 16 months). Although a histologic spectrum exists among carcinoid tumors and certain adenocarcinomas of the appendix, it is possible to delineate three biologically distinct groups. Surgical margins should be taken of all appendices because these tumors often do not form discrete masses.