Idiopathic sclerosing inflammation of the orbit is clinically characterized by an insidious, chronic and progressive fibrosing process damaging orbital structures through entrapment and mass effect. Histologically, desmoplasia and a sparse infiltrate of lymphocytes, histiocytes, plasma cells, and occasional neutrophils and eosinophils are seen. An immune pathogenesis is suspected but presently poorly understood. To characterize the inflammatory infiltrate and to compare orbital and other inflammatory fibrosing lesions, immunoperoxidase studies using the streptavidin method were performed on 16 formalin or Bouins' fixed, paraffin-embedded orbital biopsy specimens and six specimens of retroperitoneal fibrosis. Positive staining of orbital tissue occurred as follows: T-cells (UCHL-1) 94% of cases, B-cells (L26) 40%, tissue macrophages (KP-1) 56%, HLA Dr positive antigen presenting cells and activated T-cells (LN3) 44%, and immunoglobulins (kappa, 80%; lambda, 63%, IgG, 73%, IgA, 44% and IgM, 31%). Results were strikingly similar for retroperitoneal fibrosis. These findings imply a cell mediated pathogenesis in idiopathic sclerosing inflammation of the orbit that is similar to retroperitoneal fibrosis and suggest therapeutic potential for agents modifying this facet of the immune system.