Distinguishing between epithelioid peritoneal mesothelioma and papillary serous carcinomas involving the peritoneum may be very difficult, owing to overlapping morphologic features. Immunohistochemistry may facilitate establishing a correct diagnosis, but, as no single antibody has demonstrated absolute sensitivity and specificity for either mesothelioma or serous carcinoma, the differential diagnosis is based mainly on the combined use of several markers. The purpose of this study was to ascertain the sensitivity and specificity of a series of mesothelial markers [including more recently investigated antigens such as h-caldesmon (h-CD) and D2-40] and, using receiver operating characteristic curve analysis, to identify a selected appropriate panel of antibodies for differentiating between epithelioid peritoneal mesothelioma and serous papillary carcinoma of the ovary. Fifteen cases of epithelioid peritoneal mesothelioma and 40 cases of papillary serous carcinoma of the ovary (25 primary and 15 metastatic to the peritoneum) were immunostained for h-CD, D2-40, calretinin, cytokeratin 5/6, thrombomodulin, estrogen and progesterone receptors (ER and PR), Ber-EP4, B72.3, CA19-9, and CD15. h-CD and calretinin showed the highest sensitivity (100%), followed by D2-40 (93.3%) and cytokeratin 5/6 (93.3%); thrombomodulin had the lowest sensitivity (60%). h-CD and thrombomodulin had the best specificity (95%) for mesothelioma, followed by calretinin (87.5%), D2-40 (80%), and cytokeratin 5/6 (72.5%). Among carcinoma markers, ER and Ber-EP4 demonstrated the highest sensitivity (95%) followed by B72.3 (72.5%), PR (65%), CA19.9 (60%), and CD15 (45%). The specificity of the nonmesothelial markers was 100%, except for Ber-EP4 (93.3%). The relationship between the values of sensitivity and specificity of each marker using receiver operating characteristic analysis permitted the identification of h-CD, calretinin, ER, and Ber-EP4 as the markers with the best performance in differentiating epithelioid peritoneal mesothelioma from serous papillary carcinoma of the ovary.