This 12-month, double-blind, placebo-controlled study randomized 205 ulcerative colitis patients in remission to placebo or controlled-release mesalamine at 4 g/day for 12 months. Patients were stratified to either pancolitis or left-sided disease, based on previous diagnosis. Maintenance of remission was defined as a sigmoidoscopic index of < 5, less than five stools per day, and the absence of rectal bleeding. A significantly greater number of patients maintained remission on mesalamine 4 g/day than on placebo at each of five study visits, following the first one-month visit (P < 0.05). The estimated 12-month remission rates for the mesalamine group were 64% (38% for placebo, P = 0.0004). Baseline subgroups (disease location, time since last flare of active disease, and previous response to oral/rectal steroids or sulfasalazine) did not influence remission rates. Treatment-related adverse events were rare. Controlled-release mesalamine is a safe and efficacious single agent for maintaining remission of ulcerative colitis.