Mesalazine is the first line treatment in paediatric inflammatory bowel disease (IBD). There are several reports in the literature about nephrotoxicity (1/150 treated patients), from renal insufficiency to reversible nephritis. It is currently advised to follow creatinine in patients treated with mesalazine during the first 5 years of treatment; however this may reveal the problem too late. As demonstrated in this paediatric case, a high degree of clinical suspicion is needed to diagnose the side effects before irreversible kidney damage occurs. Since the treatment of IBD exacerbation and mesalazine induced interstitial nephritis is based upon steroids, delayed diagnosis of the latter is at risk.