The clinical presentation, the morphological findings, and the clinical outcome of 27 patients with biopsy-proven acute interstitial nephritis were studied. All patients except one presented with acute renal failure. Typical clinical findings were often absent. Only four patients showed the classical triad of pyrexia, rash and arthralgia. In more than half of the patients an increased blood eosinophil count was not present. Kidney biopsy is therefore needed to establish the diagnosis of acute interstitial nephritis. In many patients acute interstitial nephritis was diagnosed in the biopsy when clinically this type of kidney disease was not expected. In 17 patients renal function improved spontaneously after withdrawal of the drug responsible or treatment of the infection. In ten patients who showed further deterioration of renal function in the first 2 weeks after admission, prednisone therapy was instituted. In all of them improvement of renal function was observed, with six returning to normal.