PurposeMost malignant effusion is secondary to metastases to the pleura or peritoneum and portend poor oncological outcomes. Malignant effusion has different tumor microenvironment from primary tumor, containing a variety of cytokines and immune cells and directly contacting with tumor cells. However, the characteristic of CD4+T cells and CD8+T cells in malignant effusion remains unclear.MethodsMalignant effusion including peritoneal ascites and pleural fluid from thirty-five patients with malignant tumor were collected and compared with matched blood. A detailed characterization of CD4+T cells and CD8+T cells in malignant effusion were conducted using flow cytometry and multiple cytokines assay.ResultsThe concentration of IL-6 in malignant effusion was significantly higher than in blood. A substantial portion of T cells in malignant effusion were CD69+and/ or CD103+Trm cells. Most CD4+T and CD8+T cells in malignant effusion were exhausted T cells which expressed lower levels of cytokines, cytotoxic molecules and markedly higher levels of inhibitory receptor PD-1 compared with in blood.ConclusionOur study is the first to identify the presence of Trm cells in malignant effusion and will lay the foundation for future research on anti-tumor immunity of Trm cells in malignant effusion.