Takayasu's arteritis (TA) is a rare disease that can be difficult to diagnose in its early stage. A young woman with a fever and neck pain was thought to have TA, although computed tomographic angiography did not show any specific changes of the arteries. [(18)F]fluorodeoxyglucose positron emission tomography ([(18)F]FDG-PET) was performed to detect the source of the inflammation. Specific accumulation of [(18)F]FDG-6-phosphate in the thoracic aorta and its direct branches was observed, leading to a diagnosis of TA. [(18)F]FDG-PET is therefore considered to be useful for the diagnosis of early-stage TA.