The clinical, biologic, and pathologic features and the course and treatment of acute interstitial nephritis (AIN) due to drug hypersensitivity are reviewed. The authors report 19 additional cases of AIN, outlining some particular and unusual features. The drugs most often responsible now are penicillins and cephalosporins, cotrimoxazole, thiazide diuretics, glafenin and its derivatives, and nonsteroid anti-inflammatory agents. Diagnosis of AIN should be considered in any case of rapidly progressive renal failure occurring during drug therapy, especially when fever, skin rash, arthralgias, macroscopic hematuria, and blood or urinary eosinophilia are present. In the absence of the preceding symptoms, systematic early renal biopsy may be helpful to detect intersitial infiltrates containing lymphocytes, plasma cells, and eosinophils and/or granulomas with epithelioid cells. Immunologic tests are inconstantly positive. Their sensitivity and specificity often are doubtful in the absence of precise knowledge of the pathogenetic factors involved. Recovery may be hastened in some cases by corticoid therapy. Recurrence of the disease will be avoided by definitive suppression of offending and related drug(s).