Retroperitoneal fibrosis (RPF) - also known as Ormond's disease - is a rare progressive disease which features fibrosis (fibrous thickening) of the tissues of the lower body cavity.

It was first noted in 1905, and was subsequently 're-discovered' by Dr. J.K. Ormond in 1948. For many years it was notoriously difficult to diagnose, but now shows up clearly on MRI scans. It can sometimes be alleviated with steroid-type drugs, or by surgery.

It's cause is unknown.

[The] 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."

Source : The Lancet, vol. 367, issue 9506, pp. 241-251^{[ paywalled ]}

Note : One drug used to help treat the disorder - tamoxifen - appears to work by an as yet unknown mechanism.