We describe the case of a 45-year-old woman with a clinical history of breast cancer presenting with anuric renal failure, metabolic acidosis and bilateral grade 2-3 hydronephrosis. Following insertion of bilateral ureteral stents, urinary output was 5000 ml in the subsequent 24 hours with frankly bloody urine, after which anuria recurred. A new ultrasound examination showed hydronephrotic kidneys with properly positioned stents, a distended bladder free of clots and a hypo-anechoic, well-demarcated mass enveloping the aorta. With the echo color Doppler, injection of saline solution through a Foley catheter showed fluid flow similar to a ureteral jet within the bladder. Since the catheter balloon could not be sonographically visualized in the bladder we decided to re-examine this organ. Scans over what we thought was the bladder detected the balloon in a depleted bladder and fluid underlying it. CT urography revealed bilateral hydronephrosis secondary to a reperitoneal fibrous plaque surrounding the ureters and extending to the pelvic floor that had produced an encapsulated fluid collection. The clinical and imaging findings were strongly suggestive of acute obstructive renal failure secondary to retroperitoneal fibrosis. The retroperitoneal fluid collection, which had been mistaken for the bladder, may be due to a hematoma, aurinoma, an inflammatory process or a lymphocele.