Adenocarcinoid is a form of appendiceal carcinoid possessing features of both carcinoid and adenocarcinoma. There are two histologic types. Thirty patients had the goblet cell type, characterized by nests of large mucin-distended cells. Nine patients had the tubular type, characterized by small glandular structures lined by uniform cells. Despite abundant mucin and a goblet cell or acinar-like arrangement, a closer relationship to carcinoid than to adenocarcinoma is suggested by a concentration of tumor elements below the crypts of Lieberkuhn, a lack of evidence of neoplastic transformation of the appendiceal mucosa, and the demonstration of argentaffin or argyrophil granules in 88% of the lesions. Six tumors, all of the goblet cell type, metastasized and resulted in the death of the patients. One of the tumours that metastasized had a prominent tubular component. Most adenocarcinoids can be adequately treated by appendectomy, but hemicolectomy is recommended for those tumors showing atypical foci, a high mitotic count, or spread beyond the appendix.