['To examine carbonic anhydrase IX (CAIX), a marker of renal cell carcinoma that recently has been described in malignant effusions.', 'Pleural and peritoneal fluids with the following diagnoses-reactive (n = 23), carcinoma (n = 17), and "suspicious for mesothelioma" (n = 4)-were immunostained for CAIX, calretinin, Ber-EP4, and MOC31. A tissue microarray of epithelioid (n = 27) and sarcomatoid (n = 8) mesotheliomas and three cases of benign mesothelium were also immunostained for CAIX.', 'Mesothelial cells in both reactive (18/23) and malignant effusions (18/21) were positive for CAIX (P > .05). In carcinomatous effusions, CAIX expression was restricted to the mesothelial cells. Agreement between CAIX and calretinin expression was present in 89% of cases. In tissues, CAIX was positive in 100% of benign and 91% of malignant mesothelium.'] Does carbonic anhydrase IX ( CAIX ) differentiate between benign and malignant mesothelium?