Hibernoma, a tumor of benign brown adipose tissue can present as a large mass that can mimic a sarcoma. Imaging characterization of large extremity tumors can be very helpful in assessment of these masses for treatment planning and follow-up. We report on a case where FDG-PET imaging was performed on a patient with a large thigh mss, suspected to be a sarcoma. The FDG-PET uptake was highly avid, suggesting a preliminary diagnosis of an intermediate to high grade tumor. However, brown fat is highly metabolically active, and therefore high levels of FDG uptake can be misleading for tumor characterization in treatment planning, as it was in this case.