Between January 1979 and June 1985, 10 patients with acute allergic interstitial nephritis were seen in a clinical nephrology service at a large regional hospital. The onset of renal failure was temporally related to the use of a drug: a nonsteroidal anti-inflammatory agent (NSAID) (in four patients), cimetidine (in three), antibiotics (in two) or allopurinol (in one). The onset of renal failure was acute in three patients and insidious in seven. Two patients also exhibited marked proteinuria. Clinical features such as fever, rash, hematuria, pyuria with or without eosinophiluria, and mild to marked proteinuria had led to suspicion of the disease. The diagnosis was confirmed by renal biopsy findings of inflammatory cells, predominantly lymphocytes, plasma cells and eosinophils. Three patients required hemodialysis; two of them received steroids as well. Steroid therapy was also used in two patients with NSAID-induced proteinuria. Renal function improved in nine patients by 35 days, but one patient continued to have slow but progressive deterioration of renal function. Acute interstitial nephritis can be distinguished from other forms of acute renal failure by heavy renal uptake of gallium 67, maximal 48 hours or more after injection. The improvement in renal function after discontinuation of the implicated drug, the characteristic histopathological findings of allergic interstitial nephritis, and the presence of eosinophils and sometimes IgE in the blood suggest a hypersensitivity reaction.