An 11 year old CD-patient developed an interstitial nephritis and acute kidney failure following treatment with Mesalazine (5-ASA) and Salazosulfapyridine (SASP). After removal of the medication and treatment with hemofiltration and prednisone there was only an incomplete recovery of the renal function (creatinine-clearance 34 ml/1,73 m2/min). It is thought that an hyperergic-allergic reaction due to SASP and 5-ASA causes interstitial nephritis in inflammatory bowel disease (IBD). This reaction can be induced by re-exposition too. On the other hand IBD can be associated with glomerulonephritis. This could be a not very well known extraintestinal manifestation in IBD caused by immune-complexes in serum and glomerula. A rapid histological verification of the renal disease is necessary for successful treatment. In both renal manifestations chronic courses are possible. These observations should not lead to avoid SASP/5-ASA in treatment of IBD, but renal function should be routinely investigated.