Objective: To compare the safety and efficacy of a pH-sensitive, polymer- coated oral formulation of mesalamine (Asacol, Procter and Gamble Pharmaceuticals, Cincinnati, Ohio) with those of placebo in maintaining remission in patients with ulcerative colitis. Design: Multicenter, double- blind, placebo-controlled, randomized clinical trial. Setting: Eight private practices, five university-based medical centers, and four hospitals or clinics. Patients: 264 patients with ulcerative colitis that had been maintained in remission for at least 1 month while the patients were receiving stable doses of sulfasalazine or any oral mesalamine product. Intervention: Coated mesalamine at oral dosages of 0.8 g/d or 1.6 g/d or matching placebo for 6 months. Measurements: Treatment success, defined as maintenance of remission after 6 months, and treatment failure, defined as relapse during the study (as indicated by proctosigmoidoscopy at 1, 3, or 6 months of treatment) or withdrawal due to adverse events. Safety was assessed on the basis of laboratory analyses and patient- and investigator-noted adverse events. Results: 189 patients were compliant with the protocol for 6 months or stopped receiving therapy because of relapse or adverse events. Of these 189 patients, 25 of the 63 patients (39.7%) in the placebo group had treatment success compared with 40 of the 68 patients (58.8% [95% CI, 46.4% to 71.3%]) in the group receiving mesalamine, 0.8 g/d (P = 0.036) and 38 of the 58 patients (65.5% [CI, 52.4% to 78.6%]) in the group receiving mesalamine, 1.6 g/d (P = 0.006). In the intention-to-treat analysis of all patients, 42 of the 87 patients (48.3%) in the placebo group had treatment success compared with 57 of the 90 patients (63.3% [CI, 52.8% to 73.8%]) in the group receiving mesalamine, 0.8 g/d (P = 0.050) and 61 of the 87 patients (70.1% [CI, 59.9% to 80.3%]) in the group receiving mesalamine, 1.6 g/d (P = 0.005). Age, sex, and race were not found to predict treatment success or failure. The mesalamine tablet was well tolerated, and no clinically significant changes were seen in hematologic, hepatic, or renal laboratory profiles. Conclusion: Coated mesalamine at oral dosages of 0.8 g/d and 1.6 g/d is safe and effective in maintaining remission in patients with quiescent ulcerative colitis.