The efficacy of a capsule formulation of mesalamine was assessed in 374 patients with mild to moderately active ulcerative colitis. Patients, stratified to pancolitis or left-sided disease, received either placebo or mesalamine at 1, 2, or 4 g daily for 8 wk. Efficacy was assessed using clinical improvement--physician global assessment, sigmoidoscopic index, biopsy score, trips to the toilet, and clinical symptoms (abdominal pain, urgency, stool consistency, and rectal bleeding)--and induction of remission (more stringent criteria for physician global assessment, sigmoidoscopic index, and biopsy score). For physician global assessment of treatment benefit, 79% and 84% of patients on the 2-g and 4-g doses of mesalamine, respectively, received treatment benefit, compared with 54% on placebo (p < or = 0.0002). For the physician global assessment of treatment success, both the 2-g and 4-g doses of mesalamine were superior to placebo (57% and 59% of patients, p = 0.0021 and 0.0012, respectively), compared with 36% on placebo. Both the 2-g and 4-g doses produced statistically significant macroscopic (endoscopic) improvement compared with placebo (p < 0.004). The 4-g dose also produced a statistically significant microscopic (histologic) improvement compared to placebo (p < 0.002). Significant improvement compared to placebo was also observed at 2 g and 4 g for the four clinical symptoms and trips to the toilet (p < or = 0.003). Oral mesalamine capsules were significantly superior to placebo for inducing remission, with 29% of patients at 2 g and 29% at 4 g achieving remission by physician global assessment, compared with 12% on placebo. Forty-four percent and 48% of patients receiving 2 g and 4 g of mesalamine, respectively, achieved remission by sigmoidoscopic index (p < 0.05), compared with 31% on placebo. Thirty-nine percent of patients at 4 g daily achieved microscopic remission, compared with 23% on placebo (p < 0.03). Treatment response was not affected by extent of disease or prior steroid or sulfasalazine therapy. These data suggest that controlled-release mesalamine capsules are a safe and effective monotherapy in doses of 2-4 g daily for treating mild to moderately active ulcerative colitis, as well as for inducing remission, regardless of prior oral steroid or sulfasalazine therapy or extent of disease.