The expression patterns of PDCD4, a tumor suppressor, and β-catenin were immunohistologically investigated in gastric carcinoma tissues. In normal gastric tissues, PDCD4 was strongly expressed in the cell nuclei, but weakly expressed in the cytoplasm. In gastric adenocarcinoma tissues, nuclear PDCD4 expression was decreased, while cytoplasmic PDCD4 expression was unchanged or somewhat increased. In gastric signet ring cell carcinoma tissues, PDCD4 expression patterns were different from the expression patterns of the adenocarcinoma tissues, and PDCD4 was localized in the nuclei of the carcinoma cells as a belt in the middle of the epithelial layer. The nuclear localization of PDCD4 in the adenocarcinoma tissues was correlated with the membrane localization of β-catenin, the activation of which stimulates invasion of colon cancer cells. PDCD4 expression was correlated with β-catenin expression in gastric carcinoma cell lines, but not with E-cadherin, as the binding partner in the cell membrane.