Objective: To study the source of cells in the cell block deriving from peritoneal e usion and to analysis the value of PAX8, ER, CDX-2 and CDH17 in the di erential diagnosis. Methods: e clinical and pathological data in 146 cases of archived peritoneal e usion cell blocks were retrospectively analyzed, and the expression of PAX8, ER, CDX-2 and CDH17 were detected by EnVision immunocytochemical technique. Results: ere were 35 cases of ovarian carcinoma and 21 cases of gastrointestinal adenocarcinoma. In all the 146 cell blocks; besides, lung cancer, breast carcinoma, lymphoma and melanoma were found in these specimens. Both of the PAX8 and ER was detected in 97% (34/35) of all the 35 ovarian cell blocks, while the positive rate was 0% and 5% in the remaining samples, respectively. The positive rate in ovarian carcinomas was significant higher than the rest samples (χ2=140.56, P<0.001; χ2=116.65, P<0.001). In all of the 21 gastrointestinal adenocarcinomas, the expression rate of CDX-2 and CDH17 was 90% and 71%, respectively; from the other 125 samples, we observed 5 cases express CDX-2 and 3 cases expressed CDH17. e expression rate in gastrointestinal adenocarcinomas was signi cant higher than the other samples (χ2=140.56, P<0.001; χ2=116.65, P<0.001). Conclusion: Many malignant tumor cells have the ability to spread to peritoneal e usion, with ovarian carcinoma most common and gastrointestinal adenocarcinoma secondly; PAX8, ER, CDH17 and CDX-2 expressions are of great value in di erential diagnosis.