Retroperitoneal fibrosis encompasses a range of diseases characterised by the presence of a fibro-inflammatory tissue, which usually surrounds the abdominal aorta and the iliac arteries and extends into the retroperitoneum to envelop neighbouring structures--eg, ureters. Retroperitoneal fibrosis is generally idiopathic, but can also be secondary to the use of certain drugs, malignant diseases, infections, and surgery. Idiopathic disease was thought to result from a local inflammatory reaction to antigens in the atherosclerotic plaques of the abdominal aorta, but clinicolaboratory findings--namely, the presence of constitutional symptoms and the high concentrations of acute-phase reactants--and the frequent association of the disease with autoimmune diseases that involve other organs suggest that it might be a manifestation of a systemic autoimmune or inflammatory disease. Steroids are normally used to treat idiopathic retroperitoneal fibrosis, although other options--eg, immunosuppressants, tamoxifen--are available. The outlook is usually good, but, if not appropriately diagnosed or treated, the disease can cause severe complications, such as end-stage renal failure. Here, we review the different aspects of retroperitoneal fibrosis, focusing on idiopathic retroperitoneal fibrosis and on the differential diagnosis associated with the secondary forms.