Upper gastrointestinal tract endoscopy on admission of the patient revealed a profuse polyposis of the stomach. Histopathologic examination of one of the polyps showed a tubular adenocarcinoma of the stomach, whereas dysplasia was seen in other lesions. Staging was without evidence of lymphogenic or hematogenic metastases. On colonoscopy, no additional polyps were detected, and the areas of prior polypectomy were inconspicuous. The patient underwent DNA analysis, and a germline SMAD4mutation was found. We subsequently performed a total gastrectomy with D2 lymphadenectomy and Roux-en-Y reconstruction. The postoperative course was uneventful. Histopathologically, multiple juvenile polyps with superficial erosions were found. Intraepithelial neoplasia was observed in some polyps. Moreover, the intraepithelial neoplasia focally invaded a microinvasive well-differentiated adenocarcinoma (intramucosal adenocarcinoma). Resection margins were tumor free, and 15 regional lymph nodes showed no evidence of carcinoma metastasis. At the first follow-up 3 months after surgery, the anemia and hypoproteinemia had resolved and the patient had no gastrointestinal problems.