Retroperitoneal fibrosis is a rare autoimmune related disorder that blocks the tubes (ureters) that carry urine from the kidneys to the bladder. Retroperitoneal fibrosis occurs when extra fibrous tissue forms in the area behind the stomach and intestines. The tissue forms a mass (or masses) that can block the tubes that carry urine from the kidney to the bladder. The cause of this problem is not known. It is most common in people aged 40 – 60. Men are twice as likely to develop the condition as women. Early symptoms include: dull pain in the abdomen that increases with time, pain and change of color in the legs (due to decreased blood flow), and swelling of one leg. Later symptoms include: decreased urine output, no urine output (anuria), nausea, vomiting, changes in thinking caused by kidney failure and build-up of toxic chemicals in the blood, and severe abdominal pain with hemorrhaging (due to death of intestinal tissue). The outlook will depend on the extent of the problem and the amount of damage to the kidneys. The kidney damage may be temporary or permanent.