Reported is the case of a 71 year-old male with a poorly differentiated adenocarcinoma of the colon that had invaded the submucosal layer. A barium enema had revealed a polyp, 1.5 cm in diameter, in the descending colon, and an endoscopic polypectomy was performed. The resected specimen mainly consisted of an adenoma and a partial, poorly differentiated adenocarcinoma that had massively infiltrated into the submucosal layer. A left hemicolectomy was carried out, and no carcinoma was identified in the resected colon and/or the regional lymph nodes.