Objective: The Lynch syndrome or hereditary non-polyposis colorectal cancer (HNPCC) is a hereditary syndrome that predisposes to different types of colonic and extracolonic cancer, mainly in endometrium, stomach, ovary, and hepatobiliary and urinary tract. Occasional sarcomas have been reported in HNPCC patients. Method: We describe a pleomorphic and dedifferentiated leiomyosarcoma of the gluteus in a patient with Lynch syndrome. We study her clinical, pathology, immunochemistry and molecular alterations.