The clinical tolerance to three 5-aminosalicylic acid (5-ASA) releasing preparations (mesalazine, olsalazine and balsalazide) was assessed in a consecutive series of 43 patients with inflammatory bowel disease who were intolerant to sulphasalazine. The relative contributions to the side-effects of sulphasalazine made by its two components, 5-ASA and sulphapyridine, were also assessed in these patients. Thirty-nine (91%) patients were able to tolerate at least one of the three 5-ASA preparations. Only four (9%) patients were intolerant to all preparations, having adverse reactions previously experienced with sulphasalazine and presumably related to 5-ASA rather than sulphapyridine. The clinical tolerance to mesalazine (63%), olsalazine (70%) and balsalazide (70%) was similar, and tolerance to one drug only was found in nine (18%) patients. The commonest adverse reactions associated with 5-ASA preparations were gastrointestinal. Diarrhoea was a problem in five patients during treatment with olsalazine and three each while on mesalazine and balsalazide. Allergic reactions from 5-ASA preparations were uncommon; of ten patients with rash following sulphasalazine only one developed a rash with mesalazine. The results of this study indicate that the vast majority of patients with inflammatory bowel disease can be managed with at least one of these four 5-ASA containing preparations and that the side-effects of sulphasalazine are multifactorial in aetiology, some being due to the parent molecule, and some to one of its two metabolites, 5-ASA and sulphapyridine.