The aim of this study was to the pathogenetic backgrounds of early‐onset gastric cancers. Mutations of the E‐cadherin and β‐catenin genes were analyzed by subjecting microdissected cancer cells and corresponding non‐cancerous epithelial cells obtained from 9 gastric cancer patients under 35 years old to polymerase chain reaction‐single strand conformation polymorphism analysis. Somatic, but no germline, E‐cadherin gene mutations were detected in 6 (67%) of the patients. The cancer cells of 2 patients were exon 9‐deleted E‐cadherin molecule‐immunoreactive. Neither somatic nor germline mutations in exon 3 of the β‐catenin gene were observed in any patient. One patient lacked β‐catenin immunoreactivity and the cancer cells of 6 others showed cytoplasmic β‐catenin immunoreactivity. The E‐cadherin‐mediated cell adhesion system in the cancer cells of all the patients examined appeared to be disrupted, indicating that somatically acquired dysfunction of this system plays an important role in early‐onset diffuse‐type gastric cancers. Helicobacter pylori infection was observed in 6 (67%) of our 9 patients, an incidence higher than the average in young Japanese individuals. Thus, early‐onset gastric cancers may be attributable to environmental factors such as Helicobacter pylori infection.