Fever of unknown origin (FUO) and chronic inflammatory syndrome could be seen in many affections. The diagnostic process is still a challenge and could not be specified yet after several clinical studies performed by trained teams using two levels diagnostic procedures. For this reason, the diagnosis involves many additional and expensive tests such as computed tomography (CT scan), scintigraphic imaging, endoscopy, biopsy. There is no evidence-based recommendation for the diagnostic work-up of FUO or chronic inflammatory syndrome. Positron emission tomography with 2-[18F] fluoro-2-deoxy-D-glucose combined with CT scan (FDG-PET/CT) is widely used in malignant diseases and seems to be promising for the diagnosis of inflammatory disorders. Its role has not been yet clearly defined in the investigation of FUO and chronic inflammatory syndrome.