Retroperitoneal fibrosis is a syndrome characterised by the presence of fibrosclerotic tissue in the retroperitoneum, often encasing the ureters. In most cases, retroperitoneal fibrosis is idiopathic, but may also be associated with large vessel vasculitis at distant sites, with the so-called IgG4-related sclerosing disease, as well as with exposure to some medications, infections, malignancies, surgery, or radiation. 18-Fluorodeoxyglucose (FDG) positron emission tomography (PET) is a nuclear medicine technique which is able to accurately identify in vivo areas characterised by elevated glucose metabolism, such as inflammatory, infective, and neoplastic lesions. There is mounting evidence suggesting that FDG-PET may have a role in assessing disease activity in idiopathic retroperitoneal fibrosis, but the role of FDG-PET in secondary retroperitoneal fibrosis is less established. Herein, we present four patients with retroperitoneal fibrosis of different etiology (isolated idiopathic, associated with large-vessel involvement, associated with carcinoid tumour, and secondary to pergolide) who underwent FDG-PET as part of their workup. The implications of FGD-PET results in the diagnosis and treatment of retroperitoneal fibrosis of different etiology are discussed.