To compare the efficacy of Asacol (mesalazine coated with Eudraget-S) as a maintenance therapy with that of sulphasalazine, relapse rates of patients with ulcerative colitis and Crohn's disease, treated with sulphasalazine or Asacol were assessed in a retrospective study. A total of 164 patients were investigated, 127 on sulphasalazine and 37 on Asacol. None of the patients on Asacol was changed from sulphasalazine because of lack of efficacy to sulphasalazine. Relapse rates were measured over a 4 year period. In ulcerative colitis these were sulphasalazine 10/77 (13.0%), Asacol 5/20 (25.0%), NS; in all Crohn's disease patients, sulphasalazine 12/50 (24.0%), Asacol 11/17 (64.7%); P less than 0.0025. In patients with Crohn's disease with ileal involvement, relapse rates were sulphasalazine 9/28 (32.1%), Asacol 9/11 (81.6%), P less than 0.0125; without ileal involvement, sulphasalazine 3/22 (13.6%), Asacol 2/6 (33.4%), NS. This study suggests that Asacol is as effective as sulphasalazine in maintaining remission in ulcerative colitis and in patients with Crohn's disease without ileal involvement. Sulphasalazine seems to be more effective than Asacol in maintaining remission in patients with Crohn's disease with terminal ileal involvement.