SUMMARYA case of idiopathic retroperitoneal fibrosis, in which the fibrotic process was completely confined to the pelvis, is described. The first presenting symptom was dyspareunia but the condition was not diagnosed until serious ureteric obstruction had occurred with threatening renal failure. The physical findings then were those of a “frozen pelvis” simulating advanced carcinoma. The ureteric obstruction was relieved by uretero‐ileocystostomy, and after this operation a striking regression of the fibrosis occurred.Addendum.‐Some weeks after the completion of this paper, this case developed in an unexpected direction. The patient began to complain of dyspepsia and at the same time experienced increasing difficulty in emptying the bladder. Renal function remained unimpaired. Barium meal showed a small lesion of the stomach near the cardia with the characteristic appearance of a leiomyoma. Pelvic examination showed a recurrence of the induration and fibrosis of the pelvic floor. At cystoscopy the urethra was narrowed by the mechanical strangulation of the extrinsic fibrosis, but the bladder and bladder neck appeared normal. Repeated dilatation of the urethra failed to give relief and the establishment of an external urinostomy appeared to be the only way round the difficulty. This was done on 7th May 1964, the previously constructed ileal loop being disconnected from the bladder and brought out as a stoma in the left iliac fossa.Dyspepsia persisted, and a repeat barium meal showed such an increase in size of the lesion as to suggest a malignant process. The urinostomy functioned well, and she managed it with little inconvenience: but the bladder, although isolated now from the urinary tract, continued to fill up with infected mucus. By July the pelvic floor felt as solid and “frozen” as it had done when the patient was first seen two years earlier. Mr R. de Vere kindly saw her again and, to provide drainage of the now completely obstructed bladder, established a deliberate vesicovaginal fistula. The vesicovaginal septum at the operation was more than a centimetre thick, and tissue was taken for histological examination. This now showed an entirely different picture, with widespread infiltration of the connective tissue by an undoubted undifferentiated sarcoma.Soon after this, she became jaundiced and rapidly deteriorated, dying at home on 16th August. Permission for post‐mortem examination was unfortunately withheld.Many writers on this subject have stated that the histological diagnosis rarely offers any difficulty. Trever (1958), however, has described a case which presented as retroperitoneal fibrosis, but which ended fatally as a sarcoma, and 1 have seen a similar case. The dramatic resolution and remission seen in the present case does not accord well with a diagnosis of sarcoma ab initio, though Dr J. H. O. Earle, who has kindly reviewed the original sections including a biopsy taken at another hospital in 1961, feels that he would have been unwilling to exclude the possibility of malignancy even in the first section taken.Whether or not the lesion in this case was indeed a sarcoma from the onset cannot be established with certainty. A further lesson to be learnt from it is that cases of retroperitoneal fibrosis should be kept under close and critical review even though a long‐standing remission, either with or without treatment, seems to have been established.