Fever of unknown origin (FUO) is a diagnostic challenge, because the cause of such fever may be manifold. Studies on the use of positron emission tomography (PET) using 18F-fluorodeoxyglucose (18F-FDG), for the diagnosis of inflammation in patients with osteomyelitis or HIV have been promising and suggest its use in patients with FUO. In this study, we used FDG PET in 16 patients with FUO in whom conventional diagnostics had not been conclusive. In 12 patients, (75%) non-physiological accumulations of FDG were found which led to the final diagnosis in 11 patients (69%). FDG PET was negative in four patients (25%). Two of these patients had rheumatic fever, while in the other two patients the origin of fever could not be detected within 3 months after PET by any other laboratory or imaging means. These findings point to the high sensitivity of FDG whole-body PET for the detection of morphologically assessable foci as an origin of FUO. Moreover, they suggest a high negative predictive value of FDG PET in the setting of FUO, since in no patient with a negative FDG PET could a morphological origin of the fever be determined. In conclusion, FDG whole-body PET appears to be a promising diagnostic tool in patients with FUO, in whom conventional diagnostics had been unsuccessful.