Retroperitoneal fibrosis is idiopathic in two thirds of cases and is found most commonly as an isolated fibrotic plaque centered over the lower lumbar spine and entrapping one or both ureters. It has been postulated that the fibrosis in the idiopathic cases results from a hypersensitivity reaction to antigens leaking into the retroperitoneum from atheromatous plaques in the aorta or common iliac arteries. In the remaining one third of cases, causes include ergot-derivative drugs, retroperitoneal hemorrhage or urine extravasation, and a desmoplastic response to a variety of tumors. In up to 15% of cases, associated fibrotic processes are found elsewhere in the body. Although the disease is imaged best with CT or MR, neither of these techniques can be used to differentiate benign from malignant plaques. Histologically, the disease is characterized in its early stages by inflammatory cells and edema in a loose collagen network. The mature plaque is composed of dense fibrous tissue with minimal cellular infiltration. This progression is important as it influences MR signal characteristics.