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A diffuse peritoneal mesothelioma is a rare tumor. Exfoliative cytology forms the first step in the diagnosis of mesothelioma, since most of these cases presented with effusion. Despite well established cytomorphological features, a challenge exists in differentiating mesothelial cells, including reactive and malignant types from carcinoma cells and macrophages. Presence of “signetring” cells increases the diagnostic challenge as these can be forms of benign and malignant cells. Ancillary techniques like immunohistochemical (IHC) markers and ultrastructural analysis form useful adjunct in substantiating exact diagnosis. We report an unusual case study of a diffuse peritoneal mesothelioma in a 57-years-old lady, with no history of asbestos exposure, presenting with recurrent ascites, diagnosed on ascitic fluid cytology and on histology as an adenocarcinoma, based upon the presence of “signet-ring” cells. On review, clinicopathological correlation with IHC was helpful in forming correct diagnosis. Diagn. Cytopathol. 2010;38:435-439.