Although gastric carcinoma is one of the most common cancers worldwide, only a few histologic proximate precursors have been demonstrated. Several authors have found that foci of misplaced gastric mucosa with cystic dilatations (s.c. heterotopic mucosa) are often associated with gastric adenocarcinomas. However, adenocarcinomas originating within heterotopic gastric mucosa have never been reported. In present work, the review of 213 consecutive gastrectomy specimens in Japanese patients showed heterotopic gastric mucosa in 20.1% (n = 43). Up to 18 foci per gastrectomy were present. The heterotopic mucosa was surrounded by invaginations of the muscularis mucosae which showed strong positivity for smooth muscle actin. In 3 of the 213 specimens, an adenocarcinoma was found within a focus of heterotopic gastric mucosa. All 3 adenocarcinomas had cystic dilatations lined by neoplastic columnar epithelium with polymorphic nuclei, irregular nuclear membrane, large irregular nucleoli and pathological mitosis. The tumors had lateral bundles of smooth muscle (smooth muscle actin positive), regarded as invaginations of the muscularis mucosae. The p53 protein was strongly overexpressed in all 3 tumors. The heterotopic gastric mucosa may be one mucosal locus from which gastric adenocarcinomas may originate.