A randomized, double-blind, placebo-controlled multicentre study was undertaken to evaluate the safety and efficacy of coated, oral 5-aminosalicylic acid (Mesasal/Claversal; 5-ASA) in maintaining remission of inactive Crohn's disease for up to 12 months. A total of 248 patients were entered from eight countries, of which 206 adhered to the protocol and were included in the analysis. The patients had Crohn's disease for an average of 5 years, with their disease clinically inactive for at least 1 month prior to entry into the study, and for an average of over 12 months previously. Thirty per cent of patients had had a previous resection, 16% of patients had been treated with sulphasalazine, while none of those analysed received glucocorticosteroids. Treatment consisted of 500 mg 5-ASA t.d.s. or placebo. 'Relapse' was defined as the first occurrence of Best's Crohn's Disease Activity Index greater than 150, which had increased 60 points from the pre-trial index. The cumulative life-table relapse estimate was lower in 5-ASA patients compared to placebo (22.4% vs 36.2%, respectively, Log rank test P = 0.0395). The 12-month relapse estimate in the 5-ASA group was also lower in patients with ileal disease (8.3% for 5-ASA and 31.0% for placebo, P = 0.0535) and in patients with previous bowel resections (14.2% vs 47.0%, P = 0.0436). The incidence of side-effects was similar in both treatment groups. It is concluded that 5-ASA was significantly superior to placebo in preventing relapse of Crohn's disease; this effect was most apparent in patients with disease restricted to the ileum and in patients with previous bowel resection. 5-ASA was well-tolerated, as demonstrated by a low incidence of adverse events.