A case of malignant transformation of a gastric hyperplastic polyp with a metaplastic bone formation is reported. A large gastric polyp was revealed by X-ray examination and by endoscopy in the cardia of a 89-year-old female who had complained of nausea. A polypectomy was performed since the biopsied materials had contained some atypical glands intermingled with hyperplastic glands. Histologically, the polyp, 1.7 x 1.0 x 0.6 cm in size, was a hyperplasic polyp that was found to contain superficial dysplastic foci associated with a moderately differentiated type of a tubular adenocarcinoma. A metaplasic bone formation was found in stroma beneath the hyperplastic glandular epithelium. Thus, polypectomy of hyperplastic polyps of the stomach of more than 1 cm with dysplastic foci is recommended.