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Background and Aim: Although oral mesalamine is the first-choice drug for treating mild-to-moderate ulcerative colitis (UC), some patients show symptoms of intolerance, including exacerbation of diarrhea and abdominal pain. The present study clarified the current state and clinical courses of patients with mesalamine intolerance., Methods: Patients who were diagnosed with UC and administered oral mesalamine at eight hospitals in Japan with a follow-up period exceeding 1 year were analyzed., Results: Sixty-seven (11%) of 633 patients showed intolerance to at least one formulation of oral mesalamine. The frequency of mesalamine intolerance has increased in recent years, rising from 5.3% in 2007-2010 to 9.1% in 2011-2013 and 16.2% in 2014-2016. The most common complications were the exacerbation of diarrhea (n = 29), a fever (n = 25), and abdominal pain (n = 22). Readministration of mesalamine/sulfasalazine was attempted in 43 patients, mostly with other types of formulation of mesalamine, and more than half of these patients proved to be tolerant. The risk factors for mesalamine intolerance were female gender (odds ratio [OR] = 1.83; 95% confidence interval [CI], 1.08-3.12), age < 60 years old (OR = 2.82; CI, 1.19-8.33), and pancolitis (OR = 2.09; 95% CI, 1.23-3.60). There were no significant differences in the use of anti-tumor necrosis factor-α agents, colectomy, or steroid-free remission at the last visit between patients with and without mesalamine intolerance., Conclusions: Mesalamine intolerance is not rare, and its frequency has been increasing recently. The prognosis of patients with mesalamine intolerance did not differ significantly from that of those without intolerance., (© 2020 Journal of Gastroenterology and Hepatology Foundation and John Wiley & Sons Australia, Ltd.)