A patient with severe Crohn’s disease was treated with 5-aminosalicylic acid (5-ASA). Following initiation of treatment, serum creatinine increased slowly from 105 to 530 μmol/l (creatinine clearance 16 ml/min). The diagnosis of an interstitial nephritis was made based on normal urinary findings and the renal biopsy histology of intestitial mononuclear infiltrates and normal glomeruli, 5-ASA was discontinued and serum creatinine decreased to 245 μmol/l (creatinine clearance 40 ml/min) during the following 3 months. Partial reversibility of renal failure following discontinuation of 5-ASA and the absence of other drugs possibly causing interstitial nephritis suggest a causal relationship between 5-ASA and interstitial nephri