We performed a total gastrectomy in a 16-year-old asymptomatic CDH1 gene mutation carrier in whom two prior gastroscopies with biopsies were normal. The patient's mother died aged 39 years and her aunt died aged 21 years of gastric cancer. A germline CDH1 mutation (associated with hereditary diffuse gastric cancer) was initially identified in her mother at diagnosis and was later identified by predictive testing in this patient. Our patient is the youngest CDH1 carrier to date to have a prophylactic gastrectomy, and is several years below the age at which existing guidelines recommend consideration of gastrectomy. Multiple foci of early-stage carcinoma were found in her gastrectomy specimen. Given the family history of advanced gastric cancer in the late second decade, the unpredictable time course to development of advanced gastric cancer, and the futility of gastroscopic surveillance, we recommend consideration of prophylactic gastrectomy in adolescent asymptomatic CDH1 mutation carriers on an individual basis.