Nephrotoxicity has recently been reported with the use of 5-aminosalicylic acid (5-ASA) which has structural similarities to phenacetin and aspirin. The present paper describes 2 cases of interstitial nephritis and 1 case of end-stage failure associated with 5-ASA treatment. The first patient presented with severe renal failure which was partially reversed with 5-ASA discontinuation and steroid therapy. The second had severe renal failure (serum creatinine 469 mmol/l) but renal function stabilized with 5-ASA withdrawal. The third patient had end-stage renal failure and underwent hemodialysis and a successful kidney transplant.