Here, we report the case of fever of unknown origin (FUO) in a 77-year-old white man. The patient presented with a 3-week history of fever (between 38.5 and 39 degrees C) and general malaise. These symptoms had occurred about five to seven times during the past 30 years, and despite repeated hospitalizations, no diagnosis was made. Physical examination did not reveal any specific signs of infection nor did the patient fulfill the criteria for any rheumatic disease including vasculitides. Blood chemistry showed a greatly elevated C-reactive protein (CRP; 158.2 mg/l) and an erythrocyte sedimentation rate >100 mm, indicating an active inflammatory process, and leukocytes were significantly elevated (20,000/mul). Rheumatological parameters showed only nonspecific changes. Finally, a 2-[(18)F]-fluoro-2-deoxy-D: -glucose-positron emission tomography was performed, revealing a markedly enhanced glucose uptake in the ascending aorta and the cardiac valves, indicating vasculitis as the cause of FUO in this patient. Based on this finding, treatment was started with corticosteroids, and 2 days after the initiation of treatment, the patient had normal body temperature, and after 5 days, CRP values had returned to normal. After tapering and final complete removal of steroid treatment, the patient was still free of symptoms, hence no disease-modifying antirheumatic drug therapy was necessary.