Interstitial periodic acid-Schiff positive deposits [PASPD] of amorphous or fibrillar material were observed in 32% of [human] kidneys with a variety of tubulointerstitial diseases, including medullary cystic disease (60%), chronic pyelonephritis (20%) and hydronephrosis (33%). Immunofluorescent staining for Tamm-Horsfall protein [THP] showed interstitial localization of this protein which was identical in distribution, shape, size and fine detail with the PASPD. The deposits appeared to originate from disrupted tubules and were sometimes surrounded by inflammatory infiltrate. The intrarenal localization of THP outside of distal tubules in pathologic states indicates that this protein may have a role in the pathogenesis of renal disease.