A 47-year-old man presented with subacute, low back pain and increased creatinine levels. A CT scan showed an extrinsic, bilateral compression of the ureters by a retroperitoneal mass with pronounced uptake of 18F-fluorodeoxyglucose in positron emission tomography. Histological findings were consistent with the diagnosis of retroperitoneal fibrosis. Urological decompression was performed. The initiated corticosteroids led to a rapid clinical improvement and regression of the retroperitoneal mass. A relapse occurred after tapering of corticosteroids with a prompt response to increase of the dosage.