Hereditary diffuse gastric cancer is a rare autosomal dominant cancer susceptibility syndrome caused by germline E-cadherin (CDH1) mutations in 40% of cases with a high degree of penetrance. Screening endoscopy has not been useful in identifying early cancer, in part owing to conflicting data concerning site(s) of involvement in the stomach and the lack of endoscopically detectable pathology. Risk-reducing total gastrectomy specimens from 8 asymptomatic adults with germline mutations in the CDH1 gene (3 different pedigrees) were studied using a sequential serial sectioning protocol with submission of the entire stomach for histologic analysis. The presence, size, and distribution of signet ring cell clusters were determined for each section and geographic maps of the invasive foci were constructed and compared with gastrectomy specimens from patients with germline E-cadherin mutation and symptomatic gastric cancer. All but 1 of the asymptomatic patients with germline mutations in the CDH1 gene had negative endoscopic screening. All risk-reducing gastrectomy specimens were macroscopically normal. All contained multiple foci (mean, 10.9) of microscopic intramucosal signet ring cell carcinoma confined to the superficial gastric mucosa; no invasion of submucosa was identified. In situ carcinoma was present in 6/8 cases. The majority of signet ring foci were located in the proximal one third of the stomach, most within oxyntic-type mucosa. The number and size of foci were not related to age, but there was a trend toward more severe disease burden in women. Stomachs from the symptomatic group of patients with germline CDH1 mutations exhibited infiltrative foci with higher Ki-67 labeling that extended well beyond the superficial mucosa. In addition, while superficial signet ring cancer exhibited decreased or absent E-cadherin and β-catenin protein expression in all cases studied, deeply invasive signet ring cancer showed reversion to E-cadherin and β-catenin protein expression in a subset of mutation carriers. Our study indicates that superficial intramucosal signet ring carcinoma, although widespread, is predominantly located in the proximal one third of the stomach in patients with E-cadherin gene mutations. The observed site predilection suggests a possible role for geographically targeted endoscopic surveillance biopsy in patients who elect to delay surgical intervention.