Muir-Torre syndrome represents a rare autosomal dominant familial cancer predisposition disorder defined by the occurrence of cutaneous sebaceous tumors and an internal malignancy, most commonly gastrointestinal carcinoma. Most examples of hereditary non-polyposis cancer syndrome (Lynch syndrome), including the Muir-Torre syndrome, are associated with microsatellite instability (MSI) and germline mutations in mismatch repair genes—most commonly MLH1 or MSH2. We present a 58-year-old man with Muir-Torre syndrome and a large retroperitoneal mass (14.3 cm in greatest dimension) encompassing the left adrenal gland. Sections showed a cellular malignant tumor composed of spindle cells with a high mitotic index and lacking morphologic evidence of adipocytic differentiation. It was weakly reactive for smooth muscle actin (SMA) and negative for desmin, CD117, CD31, CD34, S100 protein and pan-cytokeratin. Further immunohistochemical analysis revealed intact expression of MLH1 but loss of MSH2 in tumor nuclei. Compared to non-neoplastic tissue, the tumor showed MSI in five of seven dinucleotide markers. Fluorescence in situ hybridization (FISH) failed to reveal 12q15 amplification, effectively excluding dedifferentiated liposarcoma as a diagnostic consideration. This is a rare case of a patient with Muir-Torre syndrome who developed a related high-grade undifferentiated pleomorphic sarcoma as the associated internal malignancy.