Malignant deciduoid mesothelioma (MDM) is a rare variant of epithelioid mesothelioma. This type of tumor might be associated with the asbestos exposure and carries a poor prognosis in general. MDM was first described by Nascimento et al. in 1994 in a peritoneal lesion of a young woman. And its diagnosis is frequently mistaken with florid mesothelial hyperplasia and peritoneal deciduosis. There are 44 MDM cases reported in the literature up today. A 59-year-old woman patient referred to our clinic was identified with an abdominal mass. Computed tomography of whole abdomen of the patient showed a mass with the widest transverse dimension of 65 × 60 mm at abdominal bifurcation in the mesenteric region. The patient was diagnosed with MDM after the cytopathological examination of the fine needle aspiration biopsy performed from the mass. Consequently, she received a total mass excision and right hemicolectomy under general anesthesia. The cytomorphological appearance of the ascitic fluid is detailed for the first time by Gillespie et al. and is described only in two manuscripts. In the present study, we aimed to report a case of a 59-year-old woman since she was diagnosed with MDM and because her cytological findings were further supported by histomorphological and immunohistochemical evaluations of the operation material obtained from the patient. © 2010 Wiley-Liss, Inc.