We assessed the tolerance and safety of two new preparations designed to release 5-aminosalicylic acid in the colon in patients with ulcerative colitis who were intolerant of sulphasalazine. Twenty-eight of 37 patients (76%) given mesalazine and 18 of 21 patients (86%) given olsalazine tolerated the new preparations with no adverse effects. No haematologic or biochemical abnormalities were detected. Adverse reactions to the new preparations were usually but not always similar to those they had previously encountered with sulphasalazine, but a few patients experienced rash and diarrhoea. In some patients intolerant of one of the new preparations, their tolerance of the other was assessed. Three patients intolerant of mesalazine tolerated olsalazine. Similarly, three other patients intolerant of olsalazine tolerated mesalazine. We conclude that not all adverse effects of sulphasalazine are due to the sulphapyridine part of the molecule. Some are due to the released 5-aminosalicylic acid and some to the parent compound. Both drugs are likely to prove useful in the management of patients intolerant of sulphasalazine.