Acute pancreatitis with severe belt-like upper abdominal pain developed within 1-4 weeks of starting medication in three patients (29-year-old man with ulcerative colitis; 43-year-old woman and 22-year-old woman with Crohn's disease) treated, for the first time, with 5-aminosalicylic acid (mesalazine), 500 mg three times daily. Concentrations of lipase initially were 545, 1182 and 3000 U/l, and of amylase 243, 449 and 129 U/l, respectively. Symptoms receded within a few hours after the drug had been discontinued, enzyme levels returning to normal in the course of the next 2-3 weeks. On repeating the drug in two of the patients, in lower dosage, the pancreatitis recurred within a few days. These observations support the view that 5-aminosalicylic acid can cause acute pancreatitis, perhaps as an allergic reaction.