Between 1980 and 1988, 12 patients at the Cleveland Clinic had biopsy-proven acute tubulointerstitial nephritis. Etiologies of the disease included drugs, systemic illness, and idiopathic causes. Clinical features were nonspecific, and the diagnosis of acute tubulointerstitial nephritis was seldom entertained in these patients prior to biopsy. Seven patients had unrelated underlying renal disease. Treatment included discontinuation of the offending agent and/or a trial of steroids. All patients had final creatinine levels lower than at diagnosis. Because the condition is potentially reversible, this disease should be considered in all patients with new azotemia who do not exhibit prerenal factors, features typical of acute tubular necrosis, red blood cell casts heralding a glomerular process, or evidence of obstructive uropathy.