Takayasu's arteritis (TA) is a rare large vessel vasculitis that is difficult to diagnose in the early stages. Therefore, it is also very difficult to manage and prevent irreversible vascular damage in TA. A 19-year-old female patient with back pain was examined using [(18)F]-FDG-PET to detect the source of inflammation. Specific accumulation of [(18)F]-FDG was observed in the thoracic and abdominal aorta, leading to the diagnosis of TA. Corticosteroid treatment resulted in clinical remission. However, the serum amyloid A (SAA) levels remained elevated. A follow-up scan showed residual uptake of [(18)F]-FDG in the thoracic aorta suggesting subclinical vascular inflammation. Methotrexate was combined with the corticosteroid, and the elevated levels of SAA became normalized. The present case suggests that monitoring serum levels of SAA and [(18)F]-FDG-PET could help clinicians to make adequate treatment adjustments in TA patients.