Chronic periaortitis involving idiopathic retroperitoneal fibrosis, inflammatory aneurysm of the abdominal aorta and perianeurysmal retroperitoneal fibrosis is a rare disease characterised by the development ofan inflammatory fibrotising process surrounding large vessels and organs of the retroperitoneum. According to some authors, chronic periaortitis results from a systemic autoimmune inflammation with vasculitis affecting aortal vasa vasorum and medium and small calibre retroperitoneal vessels. The disease is assumed to have immunopathogenetic background. Chronic periaortitis is diagnosed with the use ofultrasonography, computed tomography or magnetic resonance, and needle biopsy is mostly used for biopsy examinations. Positron emission tomography (PET) is a non-invasive diagnostic method which shows the bio-distribution of positron emitter labelled radiopharmaceuticals in the body. Due to the fact that not only tumorous, but in certain conditions also some inflammatory cells may exhibit increased accumulation of 18F-FDG (2-[18F]-fluoro-2-deoxy-D-glukose, 18F-FDG PET can be used to diagnose and to ascertain the activity of some types of chronic inflammations. We report a case ofa 60-year old man with idiopathic retroperitoneal fibrosis. 18F-FDG PET examination shoed an increased accumulation of the radiopharmaceutical around the abdominal aorta and of common ileac veins of the retroperitoneum. 18F-FDG PET fusion and computed tomography (CT) showed a correlation between the metabolically active tissue (PET) and the soft tissue infiltrate visible in CT images. A control exam done 4 months after the start of systemic therapy showed that the pathological accumulation of the radiopharmaceutical in the retroperitoneum had disappeared. 18F-FDG PET may be a useful tool to improve diagnosis of chronic periaortitis. A significant contribution of this method is the possibility to assess the activity of the pathologic process while monitoring the effect of the treatment.