Granulomatous interstitial nephritis is a rare condition whose pathogenesis is poorly understood. Of 203 renal biopsies performed between 1974 to 1994 in which interstitial nephritis was the predominant change, granulomata occurred in 12. The authors reviewed the records of these patients and performed immunopathologic and immunohistochemical studies in their biopsies to characterize the phenotype of infiltrating cells. The authors used markers for T cells, B cells, and macrophages, and determined whether they were activated through assessment of upregulation of HLA-DR molecules. Additionally, the authors attempted to delineate whether or not tubules contributed to giant cell formation through assessment of intermediate filament for keratins and macrophage markers in epithelioid cells. Drug (aspirin, gentamycin, or combination of drugs), infection (Echerichia coli or various organisms), and sarcoidosis accounted for granulomatous inflammation in three patients each, Wegener's granulomatosis and oxalosis resulting from intestinal bypass in one patient each, and in one patient the possible cause could not be determined. Except for biopsies of granulomatous inflammation resulting from infection, in which neutrophils predominated, in all other biopsies, T cells and macrophages made up most of the inflammatory cell infiltrate. HLA-DR was upregulated in mononuclear cells infiltrating the interstitium and was expressed in proximal tubular cells and endothelial cells in all but biopsies of patients with sarcoidosis. In no instance was there evidence that tubules contributed epithelial cells to giant cell formation. These findings are consistent with the notion that granulomatous interstitial nephritis is a cell-mediated form of tissue injury in which T cell-macrophage seem to play a major role.