Two cases of malignant peritoneal mesothelioma are reported. These tumors affected young women and displayed an unusual histopathologic pattern that closely simulated exuberant, ectopic decidual reaction or at least a malignant counterpart thereof. The importance of the differential diagnosis from decidual reaction is emphasized because decidua-like mesothelioma appears to be a highly malignant neoplasm. The cause of this lesion is unknown; and, considering the young age of the patients and the failure to demonstrate hormone receptors in the neoplastic cells, it is unlikely that asbestos exposure or hormonal imbalance played any role in the development of the disease.