Introduction/Objective Peritoneal malignant mesothelioma (pMM) has scarcely been reported in the context of Lynch Syndrome (LS). No cases have directly led to a diagnosis of LS. We report an unusual case of pMM presenting as a localized mass, leading to the diagnosis of LS. Methods/Case Report A 55 year old female with no prior oncologic history presented with a solitary epigastric mass. Her mother and maternal relatives had colonic, gastric and ovarian cancers. Imaging studies revealed a

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... stric mass abutting the left liver. The resected 9.8 cm mass showed a variegated histology with tubulopapillary formations and solid sheets. The neoplastic cells also showed variable cytologic features including anisocytosis, pleomorphic nuclei, multinucleation, prominent nucleoli, intranuclear pseudoinclusions, and rhabdoid features. A dense intratumoral and peritumoral lymphoplasmacytic infiltrate was present. Epithelioid pMM was confirmed with expression of cytokeratin 5/6, calretinin, WT-1, and thrombomodulin and loss of MOC31. Abnormal loss of MSH2 and MSH6 was seen (retained expression MLH1/PMS2 expression), suggesting high-level microsatellite instability. A germline pathogenic variant of MSH2 was identified. Results (if a Case Study enter NA) NA. Conclusion We report the first case of LS presenting as an atypical (unifocal) pMM showing variegated histology and lymphocytic infiltrate. Microsatellite instability in pMM is rare and has only been reported in known LS patients. Awareness of the pMM within LS is evolving and atypical features may prompt evaluation of DNA microsatellite repair system, with implications for the patient (treatment/screening) and their family.