To assess the safety and efficacy of delayed-release mesalazine (5-aminosalicylic acid) as maintenance treatment for patients with ulcerative colitis, 100 patients with quiescent colitis were randomly grouped to receive either delayed-release mesalazine or an equivalent dose of enteric-coated sulfasalazine in a 48-wk trial. Groups were comparable for age, sex, and duration and extent of disease. Relapse rates at 48 wk were as follows: sulfasalazine 38.6% (95% confidence limits, 24%-54%) and mesalazine 37.5% (95% confidence limits, 24%-53%), chi 2 = 0.01, p greater than 0.90. Mean time to relapse, cumulative relapse rate, and relapse severity were similar in the two groups. Headaches and upper gastrointestinal symptoms--common at trial entry--improved to a greater extent in patients receiving mesalazine. Delayed-release mesalazine is an effective treatment for maintaining ulcerative colitis remission and is associated with fewer side effects than equivalent doses of enteric-coated sulfasalazine.