The safety and efficacy of Claversal (coated, oral 5-aminosalicylic acid (5-ASA) 0.75 g/day) and sulphasalazine 1.5-2.0 g/day were compared for the maintenance treatment of ulcerative colitis in a 1-year double-blind trial. Three hundred and thirty-four patients, whose disease was controlled on a stable dose of sulphasalazine (1.5-2.0 g/day) for a 1-month pre-trial, entered the study. On entry, patients were assigned in a random manner to continue sulphasalazine or to switch to coated 5-ASA. One hundred and thirty-one patients in the coated 5-ASA group and 142 on sulphasalazine were analysed for efficacy. No significant difference was observed between treatments with respect to the cumulative rate of relapse. Over the 12 months, 30 (28%) of the coated 5-ASA patients versus 29 (23%) of those treated with sulphasalazine had an exacerbation of their disease (log rank test P = 0.7011). The incidence of drug-related adverse events and subsequent withdrawals was similar. The high incidence of side-effects usually associated with sulphasalazine was not observed, probably due to the fact that this population was tolerant of sulphasalazine pre-trial. Of the 37 patients who reported adverse events with previous sulphasalazine therapy, however, only two (8%) of the 24 experienced those events when randomized to coated 5-ASA while five (38%) of the 13 who continued on sulphasalazine reported those same events. Coated 5-ASA is a safe, effective therapy for maintaining ulcerative colitis in remission.