Idiopathic retroperitoneal fibrosis (IRF) is an uncommon disease of obscure etiology and pathobiology. Using sections of frozen and paraffin-embedded tissue, an immunohistochemical technique, and antibodies to a variety of macrophage- and lymphocyte-associated antigens, we studied six examples of IRF. The results showed a large population of spindle-shaped cells that expressed the immunophenotype of a tissue macrophage, that is, Leu 3a,b (CD4)+, MY7 (CD13)+, Leu M5 (CD11c)+, KP-1 and EBM-11 (CD68)+, human leukocyte antigen (HLA-DR)+, leukocyte common antigen (CD45)+, HAM-56+, and MAC387+. A subpopulation of these cells also reacted with an antibody to the "activation" antigen, interleukin 2R (CD25). A control group of "fibroblastic" lesions including keloids, desmoid tumors, and an aggressive fibromatosis displayed minimal reactivity with this panel of antibodies. The abundance of macrophages suggests that they may play an important role in the pathogenesis of IRF. If, as has been suggested by some studies, IRF is an immune-mediated phenomenon, the macrophages may be triggered to produce cytokines that stimulate fibroblast proliferation and subsequent fibrosis that characterize this disease.