Slug protein is a key regulator of epithelial-mesenchymal transition, but its expression in cancer is less well studied. To evaluate the expression of slug, E-cadherin and vimentin in adenocarcinoma cells from malignant pleural effusions, we analyzed 121 malignant pleural fluid specimens. Twenty-eight nonmalignant pleural fluid specimens were analyzed as control. Besides clinical cytological diagnosis tests, immunofluorescence, immunocytochemistry and Western blotting methods were used. Results showed strong membrane staining of E-cadherin in adenocarcinoma cells from pleural fluid. Slug mainly showed nucleus staining. Cytoplasma positive of vimentin was found in adenocarcinoma cells isolated from pleural fluid. Slug, E-cadherin and vimentin expression was found in 43/121 (36%), 87/121 (72%) and 102/121 (84%) cases, respectively. Our data showed elevated levels of slug were accompanied by down regulation of E-cadherin and the expression of vimentin in adenocarcinoma cells. In addition, there was no relationship between slug expression and patient's age or gender or tumor site. Hyperplasia epithelium cells from nonmalignant pleural fluid were uniformly negative for E-cadherin and slug. In conclusion, the results demonstrated the inverse expression of slug and E-cadherin in the majority of malignant pleural fluid cases compared with nonmalignant pleural fluid. The slug protein may be helpful to access the prognosis of patients with pleural fluid. Diagn. Cytopathol. 2013. © 2011 Wiley Periodicals, Inc.