Large cell neuroendocrine carcinomas of the colon are rare and represent only a small percentage of all colonic endocrine tumors. Here, we report a case of a colonic large cell neuroendocrine carcinomas concurrent with a colonic adenocarcinoma. A 70-year-old man presented with acute abdominal pain. A spiral computed tomography scan of the abdomen revealed eccentric wall thickening on the ascending colon. An explorative laparotomy and a right hemicolectomy were performed. Grossly, two separated masses were observed in the proximal ascending colon. One was a 7.4 × 5.1 cm ulcerative fungating lesion, and the other was a 2.8 × 1.9 cm polypoid lesion. Microscopically, the ulcerative fungating lesion showed a well-differentiated neuroendocrine morphology with necrosis and increased mitosis. Most of the tumor cells had large, vesicular nuclei with eosinophilic nucleoli, variable amounts of eosinophilic cytoplasm, and immunoreactivity for chromogranin A and synaptophysin. The polypoid lesion was a well-differentiated adenocarcinoma that had invaded the submucosa. We diagnosed these lesions as a concurrent large cell neuroendocrine carcinoma and an adenocarcinoma of the ascending colon.