Results: We present a 71 year old woman with previous colorectal, endometrium and breast cancers in a Lynch syndrome with mutations in exons 1 and 2 of MLH2, which appeared with a right gluteus mass. Grossly, it was encapsulated, whitish-yellowish with necrotic areas and measured 7,2 cm. Microscopically, the tumor was composed by a diffuse and polymorph spindle cells with fascicular pattern, focally myxoid and zones of necrosis and large hyalinization. The cells had large nuclei, sometimes giant multinucleated, with a brisk atypical mitosis activity. There was IHQ variability: positivity to desmine, MSA, calponin and negative to caldesmon and SM. There was loss of MSH2 and MSH6 repair proteins. Conclusion: We must suspect a Lynch Syndrome relational sarcomas if we found one in a patient with HNPCC in order to test to mismatch repair proteins.