The term acute interstitial nephritis (AIN) has generally been used to describe the pathologic changes observed in the kidney after a variety of ‘insults’, but many authorities have questioned whether AIN constitutes a distinct clinico-pathological entity. This report describes 5 patients in whom the diagnosis of AIN was established after ruling out other processes which could have produced similar clinical or pathologic findings. All patients presented with severe renal failure but improved both clinically and biochemically within several months. Two patients improved spontaneously and 3 improved in association with the institution of corticosteroid therapy. No etiologic agent was readily apparent in any of the patients.