Despite the promise of immunotherapy for gastric adenocarcinoma, resistance is common, necessitating the validation of new targets. Based on our previous bioinformatics analysis, the MUC3A antigen emerged as a promising candidate for immunotherapy against gastric adenocarcinoma. However, a comprehensive understanding of its expression at protein level remains elusive, despite its crucial role in determining clinical response. We also sought to establish a connection  Immunohistochemistry was used to determine the percentage of MUC3A-positive tumor cells in primary (PT) and metastatic tumor (MT) sites of 190 gastric adenocarcinoma patients. We also evaluated the association between MUC3A expression and variables such as Lauren classification, history of neoadjuvant chemotherapy and/or radiotherapy, and overall patient survival. between the expression pattern and relevant clinical variables of the disease, whenever feasible.