<b><i>Objective:</i></b> To evaluate the utility of the lymphatic endothelial marker D2-40, along with calretinin, CK5/6, desmin and MOC-31, in differentiating mesothelioma and adenocarcinoma in pleural effusion cytology. <b><i>Study Design:</i></b> Forty-five pleural effusion cases representing confirmed reactive effusions (13), mesotheliomas (11) and metastatic adenocarcinomas (21) were immunostained with antibodies against D2-40, calretinin, CK5/6, desmin and MOC-31. <b><i>Results:</i></b> D2-40 showed membranous staining in 82% of mesotheliomas and 77% of reactive effusions. Calretinin and CK5/6 were positive in 100 and 64% of mesotheliomas, and 92 and 31% of reactive effusions, respectively. All adenocarcinomas showed lack of staining with these markers. Desmin was negative in all malignant cases and positive in 85% of reactive effusions. All adenocarcinomas were positive for MOC-31 and negative for the remaining markers. <b><i>Conclusion:</i></b> Calretinin was the most sensitive in detecting mesothelial differentiation, followed by D2-40. Although useful, D2-40 necessitated cautious interpretation due to occasional focal/weak positivity, particularly in limited cellularity samples. The muscle marker desmin was useful in differentiating benign from malignant effusions but not in distinguishing mesotheliomas from adenocarcinomas. MOC-31 was both highly sensitive and specific for detecting adenocarcinoma and was useful as part of a panel of stains in differentiating cells of mesothelial origin from adenocarcinoma.