Nephrotoxicity in association with 5-aminosalicylic acid therapy is well recognised. In this patient the nephrotoxicity only became apparent after severe weight loss had occurred in association with nephrogenic diabetes insipidus. The patient aged 26 with ulcerative colitis received resin coated mesalazine (Asacol) 1200 mg/day and lost 12 kg in weight over a four month period, only complaining of thirst towards the end of this period.