A 44-year-old woman presented in March 2010 for surveillance esophagogastroduodenoscopy (EGD). In October 2004, rectal cancer had been diagnosed and treated by resection of the rectum with adjuvant chemotherapy. A diagnosis of hereditary nonpolyposis colon carcinoma (HNPCC) was established on the basis of the Amsterdam II criteria. Due to a lack of clear guidelines we decided to perform annual systematic surveillance examinations of the stomach and the most frequent tumor manifestations. Until 2009, extracolonic tumors were not observed in the patient. In March 2010, EGD showed a discrete erosive lesion in the gastric antrum, which was biopsied. Most notably, the histopathological examination revealed a poorly differentiated mucinous adenocarcinoma. Due to the poor differentiation, we decided against technically possible, endoscopic resection. The patient underwent subtotal gastrectomy and is still doing fine 28 months after surgery. This case prompted us to evaluate our surveillance approach in HNPCC patients and to review the literature.