Retroperitoneal fibrosis is a disorder in which the retroperitoneal fat is the site of a subacute and chronic inflammatory reaction and is subsequently replaced by dense fibrotic tissue. Rheumatoid nodules are chronic granulomata occurring at sites of pressure and movement, both near the body surface and internally. A 55-year-old sales-manager was admitted to radiation synovectomy after a 5 year history of excessive right and left knee effusions. There were no other clinical or laboratory abnormalities. The patient did not respond to either radioisotope synoviorthesis using radioactive Yttrium (90 Y), or to open synovectomy and prostetic surgery of the right knee. One year later, surgery of left ureter was necessary. Histological findings revealed the diagnosis of Ormond's disease. Comparative histological studies of synovial membrane of knee and retroperitoneal tissues showed local necrosis, fibrin deposition, lining cell proliferation, and infiltration by lymphocytes. Diagnosis of arthritis complicated by retroperitoneal rheumatoid nodules and retroperitoneal fibrosis was made. Serum rheumatoid factor has been negative. For the last 3 years, the patient has been on successful therapy with azathioprine. Rheumatoid nodules of the retroperitoneum have vanished completely and frequency of knee effusions decreased.