This 17-year-old woman had chronic congenital lymphedema in the left lower extremity since childhood. She underwent surgeries to remove excessive lymphedematous tissues more than 15 times previously. Histopathology of the specimen from the recent surgery revealed angiosarcoma; therefore, FDG-PET/CT scan was arranged to determine the extent of tumor spread, and distant metastases were discovered. Stewart-Treves syndrome is angiosarcomas that arise secondary to chronic lymphedema. Because of the high lethality of this condition, the FDG-PET/CT scan may be a clinically useful imaging modality to detect the possible malignant transformation earlier for patients with chronic lymphedema.