Carcinoid tumors display a wider histologic spectrum than was previously thought. Some tumors may show atypical features such as glandular profiles, a spindle cell pattern, squamous or osteoid metaplasia, or pleomorphism. In rare cases they may be poorly differentiated and resemble undifferentiated carcinoma or lymphoma. As is well known, some carcinoids are associated with well-defined syndromes, such as the carcinoid or the Zollinger-Ellison syndrome, due to the secretion of amines or peptides. Immunohistochemical analysis of these tumors, however, has shown that whereas one amine or peptide may predominate, most are multihormonal. These findings are also seen with the clinically silent tumors such as the foregut and hindgut carcinoids. Furthermore, the immunohistochemically demonstrated amines and peptides in the primary tumor do not necessarily correspond to those normally found in the overlying endocrine cells. An increasing number of tumors have recently been described which contain an admixture of neoplastic endocrine and nonendocrine epithelial cells. Thus, the classification of gut mucosal tumors into carcinomas and endocrine tumors has had to be modified to include those tumors which have been designated as mixed or composite tumors. These have been further subdivided into several distinctive histologic types. Some of these tumors, such as the microglandular-goblet cell carcinomas, have a distinctive clinical behavior, whereas others, such as the adenoendocrine cell carcinomas, appear to behave in a manner similar to adenocarcinoma. Additionally, there is another tumor type, namely the amphicrine tumor, which differs from the mixed tumors in that endocrine and epithelial cell constituents are present within the same cell.