A 60-year-old female patient was admitted to our hospital in April, 2010 because of low-grade fever and malaise for several months. Physical examination on admission revealed no abnormalities except for a body temperature of 37.2°C. Blood examinations showed moderate anemia and a high erythrocyte sedimentation rate. There were no other specific abnormal findings. A systemic CT scan study disclosed diffuse thickening of the artery wall through the ascending, descending and abdominal aorta to the bilateral iliac arteries. In order to evaluate the quality of the vessel lesions, a FDG-PET/CT study was performed and revealed abnormal accumulation of 18F-FDG in the thickened wall, suggesting an inflammatory process in the lesion. Taking all these findings into consideration, we made the diagnosis of Takayasu's arteritis, and treated the patient with prednisolone. The treatment was effective and her symptoms improved. A later CT scan revealed that the artery wall became somewhat thinner. Takayasu's arteritis is a disease whose diagnosis is difficult to make because there are neither specific signs nor diagnostic laboratory findings in its early stage. We found that FDG-PET/CT was helpful in the diagnosis and evaluation of lesions in a patient with Takayasu's arteritis.