Background: In the last 10 years several reports have linked oral 5-aminosalicylic acid (5-ASA) therapy to acute and chronic tubulointerstitial nephritis, but to our knowledge only 2 patients have been reported with terminal end-stage renal disease due to mesalazine (5-ASA). After 1 year of treatment with 5-ASA annual monitoring of serum creatinine is recommended. Results: We report the development of end-stage renal disease in a patient 10 months after the last assessment of serum creatinine, which was normal at 71 µmol/l. Conclusion: Because of this dangerous side effect, we would like to recommend 6-monthly assessment of renal function.