Acute interstitial nephritis is an important cause of acute renal failure resulting from immune-mediated tubulointerstitial injury, initiated by medications, infection, and other causes. Acute interstitial nephritis may be implicated in up to 15 percent of patients hospitalized for acute renal failure. Clinical features are essentially those of acute renal failure from any cause, and apart from a history of new illness or medication exposure, there are no specific history, physical examination, or laboratory findings that distinguish acute interstitial nephritis from other causes of acute renal failure. Classic findings of fever, rash, and arthralgias may be absent in up to two thirds of patients. Diagnostic studies such as urine eosinophils and renal gallium 67 scanning provide suggestive evidence, but they are unable to reliably confirm or exclude the diagnosis of acute interstitial nephritis. Renal biopsy remains the gold standard for diagnosis, but it may not be required in mild cases or when clinical improvement is rapid after removal of an offending agent or medication. The time until removal of such agents, and renal biopsy findings, provide the best prognostic information for return to baseline renal function. Corticosteroids appear to provide some benefit in terms of clinical improvement and return of renal function, but no controlled clinical trials have been conducted to confirm this.