Periureteric fibrosis, an idiopathic retroperitoneal fibrotic process, is a recently recognized disease characterized by unilateral and bilateral ureteral obstruction. Ormond (1), who first described this condition in 1948, reported 2 cases of bilateral ureteral obstruction. In 1954, Chisholm et al. (2) published a case, postulating the etiology and suggesting methods of management. In 1955, Raper (3) reviewed the literature and cited 3 instances in which oliguria developed from bilateral ureteral obstruction. He believed that the disease was of vascular origin. The present report proposes to describe certain clinical and radiographic features observed in 4 cases of periureteric fibrosis studied at the Massachusetts General Hospital. It is hoped that the observations may facilitate the early diagnosis of this rare syndrome. Pathology Periureteric fibrosis is a progressive process involving the retroperitoneal tissues. Raper suggests that it originates in the region of the great vessels, while Chisholm believes that it takes its origin from the fascia of the psoas muscle. It has been found in a case of periarteritis nodosa (4). Trever (5) presented an example in association with reticulum-cell sarcoma and urged extensive search for an etiologic factor. It is known that regional ileitis, appendicitis, and diverticulitis are inflammatory processes capable of producing ureteral obstruction. Chisholm and his associates suggested that in their patients infection in the retroperitoneal space, which encloses the aorta, inferior vena cava, and the urinary tracts, could produce the fibrotic changes. In many of the reported cases, however, evidence of inflammation was absent (6). Whatever its origin may be, it is known that the fibrotic process extends from the kidneys to the sacral promontory, spreading laterally to involve one or both ureters (1, 2, 3, 7). In one case at the Massachusetts General Hospital, the fibrosis involved the right pelvic ureter, extending medially to the left of the midline over the sacrum and just over the pelvic brim. The ureter embedded in the fibrous tissue was extrinsically narrowed but the ureteral mucosa was intact. The cut surface of the fibrotic lesion is grayish-white, its appearance being similar to that of metastatic carcinoma or lymphoma (8). Microscopically the process shows an actively sclerosing type of inflammatory fibrosis infiltrated with monocytes, lymphocytes, and some eosinophils. Culture for organisms is negative. Amselem (9) commented that the retroperitoneal cicatrix may be due to surgical trauma and the presence of suture material. It is interesting that in one of Ormond's cases, in MacLean's (10) case, and in the 4 here presented there had been previous abdominal surgery, but it is difficult to determine whether surgical trauma bears any causative relationship to the fibrotic process, whose etiology is unknown. Clinical Features