To clarify the significance of E‐cadherin gene alterations in the development of diffuse‐type adenocarcinoma of the stomach, we analyzed mutations of the E‐cadherin gene using the polymerase chain reaction single‐strand conformation polymorphism (PCR‐SSCP) method followed by direct sequencing. Twenty‐two signet ring cell carcinomas of the stomach (10 intramucosal and 12 advanced cancers) were examined. Genomic DNA was extracted separately from cancerous and non‐cancerous tissues and PCR‐SSCP analysis was performed on exons 5 to 9 and the adjacent 30‐ to 40‐base‐pair intron sequences of the E‐cadherin gene. Mobility shifts were found in 2 of the 10 intramucosal cancers. In 2 of the 12 advanced cancers, abnormalities of the E‐cadherin gene were observed in intramucosal lesions as well as in deeply invaded areas. These results indicated that E‐cadherin gene mutations are an early event in the development of signet ring cell carcinoma of the stomach. Direct sequencing revealed that the locations of mutations of the E‐cadherin gene included the branch point sequence in the intron which is responsible for RNA splicing. Reverse transcriptase‐PCR demonstrated aberrant RNA splicing in a case with a branch point mutation, suggesting that branch point mutations play an important role in functional modifications of E‐cadherin in signet ring cell carcinoma of the stomach.