BACKGROUNDClaudin 18.2 (CLDN18.2) is a cell surface protein expressed by gastric cancer cells. The monoclonal antibody zolbetuximab binds CLDN18.2-positive cancer cells and causes cancer cell death. A few studies researched the prognostic effect of CLDN18.2 expression in metastatic gastric adenocarcinoma.AIMTo identify the prognostic value of CLDN18.2 expression in patients with metastatic gastric adenocarcinoma.METHODSThis study was conducted with 65 patients over the age of 18 who were diagnosed with metastatic gastric adenocarcinoma. We investigated the effect of CLDN18.2 expression on clinicopathological characteristics (age, sex, histological grade, Lauren classification, family history, metastatic site, HER2 expression) and prognosis for patients with metastatic gastric adenocarcinoma.RESULTSCLDN18.2 expression was positive in 73.8% (48) of the patients. During the median 17.7-mo follow-up period, 89.2% (58) of the patients died. Median progression-free survival and overall survival (OS) were 6 mo (95% confidence interval: 1.6-10.4) and 12 mo (95% confidence interval: 7.5-16.5). There was no statistically significant correlation between CLDN18.2 expression and clinicopathological characteristics of the patients. In univariate and multivariate Cox regression analysis, there was no correlation between clinicopathological characteristics of patients and progression-free survival or OS.CONCLUSIONCLDN18.2 expression was quite high in patients with gastric adenocarcinoma, identifying the proportion of the patients in whom zolbetuximab would be efficacious. There is no statistically significant correlation with clinicopathological characteristics and OS. CLDN18.2 is not a prognostic marker in patients with gastric adenocarcinoma, although it is predictive.