• The development of acute renal failure and interstitial nephritis due to therapeutic doses of nonsteroidal anti-inflammatory drugs has been documented repeatedly in adult patients but is rare in children. We report the occurrence of this complication in a child. Acute renal failure and hyperkalemia developed in a 2-year-old boy with juvenile rheumatoid arthritis after one month of naproxen sodium therapy. The evidence of renal toxic effects became manifest after an episode of dehydration. A percutaneous renal biopsy specimen revealed interstitial nephritis. The patient recovered promptly after withdrawal of the drug.