In five patients (4 men aged 34, 20, 22 and 29 years, and a woman of 34 years) tubulo-interstitial nephritis developed during treatment with 5-aminosalicylic acid (5-ASA) containing drugs for colitis ulcerosa, proctocolitis or Crohn's disease. Two patients had an impaired renal function before treatment with 5-ASA medication. In all patients there was only an incomplete recovery of renal function after cessation of the therapy. It is therefore necessary to monitor renal function regularly in patients receiving 5-ASA containing preparations.