Nine episodes of drug associated acute interstitial nephritis, in seven patients, were treated between 1972 and 1980. The drugs implicated were cotrimoxazole (three times), ampicillin, Magnapen (ampicillin and flucloxacillin), penicillin, gentamicin, paracetamol and bendrofluazide. The time from exposure to the onset of symptoms ranged from one to 30 days. Presentation was with acute renal failure, which was non-oliguric in five cases, accompanied by rash (four), fever (four), and loin pain (two). Renal biopsy was carried out in all cases, and showed a characteristic interstitial infiltrate comprising substantial numbers of lymphocytes and plasma cells, with a variable number of neutrophils, eosinophils and histiocytes. Immunofluorescence was negative in all four cases studied in the acute phase, and showed scattered deposits of IgG, IgM, IgA and C3 on the tubular basement membrane in one patient during recovery. Significant proteinuria and an abnormal urine deposit were present in all cases, and seven of nine had radiological evidence of enlarged kidneys. Seven episodes were treated with high doses of methyl prednisolone and in all there was a response with a diuresis or spontaneous fall in serum creatinine within 72 hrs, and recovery of virtually normal renal function. Of two cases who did not initially receive steroids, one improved more slowly and one developed chronic renal impairment.