The immunohistochemical diagnosis between epithelial mesothelioma and adenocarcinoma is currently based on the use of a panel of antibodies to adenocarcinoma-associated antigens and a few antibodies to mesothelial-associated antigens. Since the introduction of epitope retrieval methods, the sensitivity of many antibodies has been enhanced. Thus, a reevaluation of the mesothelioma/adenocarcinoma diagnostic panel becomes necessary. We studied 268 paraffin-embedded formalin-fixed tumor samples that included 57 epithelial mesotheliomas and 211 adenocarcinomas of various origins, comparing an extensive antibody panel with and without heat-induced epitope retrieval (HIER). Marked increase in the sensitivity of several antibodies, with no loss of specificity, was found when HIER was used. After statistical analysis, the antibodies to the epithelial glycoproteins carcinoembryonic antigen, BerEp4, and Bg8 emerged as the best discriminators between adenocarcinoma and epithelial mesothelioma within the entire panel. The mesothelium-associated antibodies, HBME-1, calretinin, and thrombomodulin were less sensitive and less specific than the former, although they were found to be useful on certain cases. Antibodies to cytokeratins and vimentin, although of minor diagnostic value in this context, may be helpful to evaluate the quality of antigen preservation. This study confirms the value of immunohistochemistry to accurately distinguish mesothelioma from adenocarcinoma when an antibody panel approach is used. The addition of heat-induced epitope retrieval methods increases the effectiveness of the procedure and is recommended for most of the antibody panel members.