We analyzed the diagnostic role of claudins in effusion cytology in 325 effusions, including 218 ovarian, 49 breast, 15 cervical or endometrial, 10 gastrointestinal, and 8 lung adenocarcinomas and 25 malignant mesotheliomas (MMs). Specimens were analyzed for claudin-1 and claudin-3 expression using immunohistochemical analysis. Ovarian and breast adenocarcinoma were further analyzed for claudin-7 expression. Claudin-1 expression was most frequent in ovarian and cervical or endometrial adenocarcinoma compared with other adenocarcinomas and MMs (P < .001). Claudin-3 expression was comparable in adenocarcinomas of different origin but was absent in MMs (P < .001). Reactive mesothelial cells rarely expressed claudins. Claudin-7 expression was higher in ovarian than in breast adenocarcinoma (P < .001). Our data suggest that expression of claudin-3 or claudin-7 is specific for adenocarcinoma and rules out the diagnosis of cells as mesothelial and that absence of claudin-1 expression excludes ovarian carcinoma as the possible origin of metastatic adenocarcinoma. Claudins may, therefore, be of diagnostic value in effusion cytology.