Retroperitoneal fibrosis (RPF) is a rare disease characterised by fibrous tissue proliferation in the retroperitoneum, with encasement of the ureters and large vessels of the abdomen as the most destructive of potentially severe complications. It can either be idiopathic, or secondary to infections, malignancies, or the use of certain drugs. The idiopathic form accounts for approximately 75% of the cases, and is usually responsive to immunosuppressive therapy. In recent years, the emergence of a new clinical entity, IgG4-related disease (IgG4-RD), shed light on many fibro-inflammatory disorders once thought to be separate clinical entities, although frequently associated in the so-called multifocal fibrosclerosis. Among these, together with sclerosing pancreatitis and cholangitis, pseudotumour of the orbit, idiopathic mediastinal fibrosis and other conditions, is idiopathic retroperitoneal fibrosis (IRF). Both IRF and IgG4-RD can be associated with a wide variety of disorders, usually governed by immune-mediated (and particularly auto-immune) mechanisms. In our review, we discuss the clinical and therapeutic challenges IRF presents to the internist, as well as the meaning of its recent inclusion in the IgG4-RD spectrum from a clinical practice standpoint.