We report the case of a 76-year-old woman with a flat elevated lesion (laterally spreading tumor; LST) in the transverse colon that had been identified 2 years before the current presentation. Pathologically, the lesion had been diagnosed as a serrated adenoma with severe dysplasia. The patient had refused endoscopic resection because she was afraid of the risk of colonic perforation; she did not come to follow-up until 2 years after the diagnosis, when she had advanced colon cancer in the transverse colon where the LST had been detected. Histopathological examination confirmed a poorly differentiated adenocarcinoma with no adenomatous component such as had been found in the LST lesion.