Chronic expanding hematoma is a rare presentation of a hematoma characterized by a persistent increase in size for more than a month after the initial hemorrhage. We present a 65-year-old man with a chronic expanding hematoma in his ilium who was receiving anticoagulant treatment. The patient had a delayed manifestation of a femoral neuropathy with massive bone destruction. 2-Deoxy-[18F]fluoro-D: -glucose (FDG) positron emission tomography (PET) imaging revealed an increased uptake in the rim of the mass in images acquired 1 h after FDG injection. FDG-PET scans were performed using a dedicated PET scanner (HeadtomeV/SET2400 W, Shimadzu, Kyoto, Japan), and the PET data for the most metabolically active region of interest (ROI) were analyzed. The maximum standardized uptake value (SUVmax) was set to a cut-off point of 3.0 to distinguish between benign and malignant lesions. The SUVmax of the patient's lesion was 3.10, suggesting a malignant lesion. The characteristics of FDG-PET images of chronic expanding hematomas, including the uptake of FDG in the peripheral rim of the mass as a result of inflammation, should be recognized as a potential interpretive pitfall in mimicking a sarcoma.