The patient was a 61-year-old man who developed gossypiboma of the left thigh and femur resulting in the imaging appearances of a malignant surface tumor. He had a past history of surgery on the left femur for open fracture 40 years previously. Radiographs and CT showed a soft tissue mass with osteolysis and periosteal thickening of the left femur. On MRI, the mass showed heterogeneous signal intensity with contrast enhancement at the periphery, suggesting a malignancy. 99mTc-HMDP bone scintigraphy showed a faint ring-like uptake, but thallium -201 scintigraphy did not show any uptake in the tumor. An extensive intralesional excision was performed. Postoperative histopathological examination showed a fibrous foreign body with reactive changes. There were neither viable cells nor atypical giant cells around the foreign body. No malignant change was evident. Based on surgical and histopathological examinations, the tumor was finally diagnosed as gossypiboma related to a retained surgical sponge.