Takayasu's arteritis (TA) is characterized by a chronic inflammatory arteriopathy with unknown etiology affecting large vessels. As TA occurs mostly in young females, elderly males with this disease may be under-diagnosed. A 65-year-old Chinese male with a 6-week history of fever, fatigue, low appetite, night sweats, and a 5-kg weight loss did not experience symptoms of jaw claudication, localized headache, sudden visual change, upper or lower extremity claudication, arthralgia, and rashes. On physical examination and laboratory findings, there was no evidence of infectious disease or malignancy. F-18-Fluorodeoxyglucose positron emission tomography/computed tomography (F-18-FDG PET/CT) was performed which suggested TA. The patient was treated with methylprednisolone and clopidogrel. His symptoms subsided gradually and hematological examination showed decreases of erythrocyte sedimentation rate and high-sensitivity C-reactive protein. We reported here a patient who presented with fever of unknown origin and was diagnosed with TA using PET/CT. Although elderly males are beyond the predominant population of TA, TA should be considered when there is no evidence of infectious disease and malignancy.