BACKGROUND: Neuroendocrine cell tumor in adenoma of the sigmoid is a rare neoplasm coexistence and it is considered as a mixed glandular-neuroendocrine neoplasm.DESCRIPTION OF CASE: An 84-year-old woman underwent surgical removal of a tumor located in the sigmoid, diagnosed as adenocarcinoma on a previous biopsy. On gross examination, apart from the ulcerated tumor, a polyp measuring 2 cm was observed, which histologically corresponded to a villotubular adenoma. In two sites of the adenoma, solid nests of smaller cells were observed, having small amount of cytoplasm, round nuclei with finely stippled chromatin. Mitoses were abundant. These cells were located in the lamina propria and muscularis mucosa, without disturbing the polyp architecture, and showed immunophenotypic characteristics of neuroendocrine carcinoma (NEC). The histologic findings set the diagnosis of mixed adenoma and NEC. The patient remains free of recurrence or metastasis by NEC, after two years of follow up.CONCLUSION: The recognition of NEC in an adenoma will help to avoid potential diagnostic pitfalls. Mixed adenoma and NEC is rare, with uncertain biological behavior. This case reinforces the view that NECs without infiltration of submucosa may have a better prognosis.