Over a 12-month period, we diagnosed poorly differentiated infiltrative independent-cell gastric adenocarcinoma in two brothers and one sister aged 41 to 47 years. Their father had died from antral cancer at the age of 34 years. These cancers had two characteristic clinical features: rapid course and distant malignant dissemination. In all three patients, polymerase chain reaction-sequencing of the E-cadherin (CDH1) gene of white blood cells identified a heterozygous nonsense mutation of exon 3, producing a stop codon at position 95 (Q95X), resulting in a truncated protein. The alteration of this protein, which plays a crucial role in epithelial cell adhesion, probably explains the clinical expression in this type of familial diffuse gastric cancer.