Background: Mixed endocrine-exocrine tumors display histologic features of endocrine and glandular differentiation. Unlike in collision tumors, the two components are thought to arise from a monoclonal precursor. Evidence from molecular testing supports the monoclonal theory and suggests that the exocrine component may give rise to the endocrine component but not vice versa.

Case report: We report a case of an adenomatous polyp in the large intestine that had groups of endocrine cells arising from the crypt bases of the adenomatous (exocrine) epithelium. To our knowledge, ours is only the second report of an adenomatous polyp in which groups of microcarcinoid endocrine cells were recognized. The histologic findings in our case correlate with the molecular findings described in mixed endocrine-exocrine tumors.

Conclusion: Our description may represent the primordial stage of a mixed endocrine-exocrine neoplasm.