Idiopathic retroperitoneal fibrosis (sclerosing retroperitonitis).

Three cases of idiopathic retroperitoneal fibrosis, one of which was localized to the perirenal area, are presented. The predominance of plasma cells, which may be difficult to recognize because of distortion unless methyl green-pyronine staining is done, and the character of the fibrous tissue indicated the non-neoplastic nature of the processes. This diagnosis was confirmed by immunoperoxidase studies that demonstrated polyclonality of the lymphoplasmacytic component. Immunologic studies, which may be performed on paraffin-embedded tissue, are helpful in differentiating this lesion from the sclerosing lymphomas that also occur in the retroperitoneal area.