The patient was a 57-year-old Japanese woman who had been identified as having anemia and hypoproteinemia by a local group medical check-up. Esophagogastroduodenoscopy revealed an elevated lesion of 35mm in diameter on the posterior wall of the gastric antrum, surrounded by multiple polyps. The elevated lesion was diagnosed as an adenocarcinoma on the basis of biopsy, and total gastrectomy was performed. Histological examination of entire resected stomach revealed two lesions of intramucosal carcinoma together with multiple hyperplastic polyps. To investigate the mucin phenotypes and carcinogenesis of these lesions, immunohistochemical analyses of MUC2, MUC5AC, MUC6, CD10, Ki-67 and p53 protein expressions were performed in 17 hyperplastic polyps and two cancerous lesions. Expression of the MUC6 positive glands beneath the surface foveolar epithelium of hyperplastic polyps caused a morphological change from sessile to pedunculated, suggesting that this was also involved with cancerous changes. The case reported herein seems to be extremely interesting in terms of elucidating the process whereby gastric cancer arises from hyperplastic polyps.