To define interstitial nephritis without preselection bias, 25 consecutive renal biopsy specimens from patients with tubular damage, interstitial damage and interstitial inflammation were analyzed in detail. In four patients (all with acute renal failure), tubulitis, and interstitial eosinophil and lymphocyte infiltration were found, but no glomerular abnormalities. In four others, the findings were similar but some glomerular abnormalities were noted. Two patients had probable healed interstitial nephritis. The clinical presentation varied from transient renal insufficincy to oliguric renal failure. Three of the patients with glomerular abnormalities had significant proteinuria. When the 10 patients with interstitial nephritis were compared with the other 15 serving as controls, striking features in the former group were skin rash, eosinophilia, the absence of hypertension and the frequency of administration of penicillin and its analogs. Serum immunoglobulin E (IgE) levels were elevated in three of the patients. The striking eosinophilia, interstitial eosinophil infiltration and increased IgE levels suggest that allergen-reaginic complexes may be involved in the pathogenesis of the lesion.