Whether chronic interstitial nephritis (pyelonephritis) mainly results from kidney infection is widely debated. We studies 101 patients with interstitial nephritis, selected from 320 patients with newly diagnosed chronic renal disease, for frequency of etiological factors. Eleven had no etiologic factor(s) identified; 89 had clearcut factor(s): anatomic abnormalities 31, analgesic abuse 20, hyperuricemia 11, nephrosclerosis 10, stones 9, sickle cell disease1, tuberculosis 1, multiple causes7. Bacterial infection (present in 27%) was found only with another preceding primary cause of renal damage. Analgesic abusers frequently denied drug ingestion; 15% had urinary tract infection and 20% classical papillary necrosis. Two had family histories of analgesic abuse with nephropathy. We conclude that interstitial nephritis is a common form of chronic renal disease, is seldom idiopathic, rarely results from bacterial infection alone in adults, and frequently results from analgesic abuse in the United States.