Within 30 months the diagnosis of drug-induced acute interstitial nephritis was made in ten patients with acute onset of renal failure of clinically unknown cause. Allergenic substances were discovered to be antibiotics, pyrazol and indol derivatives, piromidic acid and chlorazanil. In contrast to the known course of methicillin nephritis the clinical signs were undramatic. Non-oliguric renal failure predominated, sometimes with leucocyturia, microhaematuria and moderate proteinuria. Intermittent haemodialysis was necessary in half the cases. Renal function developed favourably without further specific treatment, however, plasma creatinine did not return to normal levels in most cases. Percutaneous renal biopsy was the definitive diagnostic step. Indications for biopsy in cases of unclear acute renal failure should thus be handled liberally in order to prevent continued drug exposure with the danger of irreversible renal failure.