AbstractThe distinction between malignant mesothelioma and adenocarcinoma is a diagnostic challenge in cytologic specimens of effusion fluids. As for today, no single antibody has demonstrated absolute sensitivity or specificity for Mesothelioma. D2‐40 and podoplanin have recently been recognized to stain mesothelial cells. Our aim for this study was to evaluate the utility of these two markers as indicators of mesothelial cells using cell blocks by comparison with two other established mesothelial markers. A total of 40 cell blocks of effusion fluids including cases of epithelioid mesotheliomas, metastatic carcinomas and benign cases with reactive mesothelial cells were selected. A panel of immunostains including D2‐40, podoplanin, CK5, and calretinin was performed. D2‐40 and podoplanin were positive in 100% of mesothelioma cases in comparison to metastatic adenocarcinoma cases where the positivity was 0%. It is concluded that D2‐40 and podoplanin are very useful markers for mesotheliomas. Since these markers are extremely helpful in differentiating epithelioid mesothelioma from metastatic adenocarcinoma, they shall be a valuable addition to the battery of markers used to differentiate the two entities. Diagn. Cytopathol. 2007;35:342–347. © 2007 Wiley‐Liss, Inc.