The differential diagnosis of pleural mesotheliomas and lung adenocarcinomas presents a continued challenge in the practice of surgical pathology. Paraffin immunohistochemistry (IHC) using different panels of antibodies can be helpful in some cases, but, as yet, no antigen is expressed specifically in mesotheliomas nor in adenocarcinomas. Using well characterized monoclonal antibodies (MAb) that recognized distinct mesenchymal and epithelial adhesion proteins, N-cadherin (13A9 MAb) and E-cadherin (E9 MAb), respectively, we found previously that in frozen-section IHC mesotheliomas and adenocarcinomas had distinct cadherin phenotypes: mesotheliomas were positive for N-cadherin, and lung adenocarcinomas were positive for E-cadherin. Using antigen-retrieval methods, we successfully extended our study to formalin-fixed, paraffin-embedded tissue sections. Tumors from 28 patients (14 originally diagnosed as mesotheliomas, and 14 diagnosed as adenocarcinomas) were stained with 13A9 MAb and E9 MAb. Review of hematoxylin-eosin sections excluded from analysis one case previously diagnosed as mesothelioma, which represented a hemangiopericytoma. Of the remaining 27 cases, 12 of 13 mesotheliomas were positive for N-cadherin and negative for E-cadherin. The exception was a multifocal microscopic papillary tumor of apparent mesothelial origin, which was negative for both N-cadherin and E-cadherin. Conversely, 13 of 14 adenocarcinomas were E-cadherin positive and N-cadherin negative except for one adenocarcinoma with focal N-cadherin expression. One case of a poorly differentiated adenocarcinoma invading skeletal muscle was negative for both 13A9 and E9. These studies confirmed the utility of the cadherin antibodies in distinguishing pleural mesotheliomas from lung adenocarcinomas. The reactivity of the cadherin-specific antibodies with antigens in paraffin sections make them powerful and reliable markers in the practice of diagnostic surgical pathology.