A randomized, placebo-controlled multicenter trial was conducted to evaluate the efficacy and safety of a delayed-release formulation of 5-aminosalicylic acid (5-ASA) (Asacol; Giuliani & Bracco, Milan, Italy) for prevention of clinical relapse in 125 patients with inactive Crohn's disease. Patients in remission [Crohn's Disease Activity Index (CDAI) less than 150] between 3 months and 2 years were randomly allocated to receive either 800 mg 5-ASA three times daily (n = 64) or placebo (n = 61) for up to 12 months or until relapse of symptoms. Relapse was defined by a CDAI greater than 150, with a minimum increase of 100 points over the baseline value. The cumulative relapse rates were 12% in the 5-ASA group and 22% in the placebo group at 3 months [95% confidence interval (CI) for the difference, -4 to 24]; 28% and 41%, respectively, at 6 months (95% CI, -4 to 30); and 34% and 55%, respectively, at 12 months (95% CI, 3-39; P = 0.02, log rank test). Significant decrease in the risk of relapse was found in patients with ileitis, in those with previous bowel resection and, in those with prolonged prestudy remission. Eight patients (5 on 5-ASA, 3 on placebo) withdrew from the study because of adverse reactions, but no major clinical or laboratory adverse effect was observed. It is concluded that oral 5-ASA coated with Eudragit S (Rohn Pharma GmbH, Wieterstadt, Germany), 2.4 g daily, is safe and seems superior to placebo in preventing or delaying clinical relapse in Crohn's disease, especially in milder cases and in ileal disease.