BACKGROUND 5-Aminosalicylate (5-ASA) is widely used as the first-line drug for ulcerative colitis (UC). 5-ASA is mostly a safe and effective drug, but it can bring about exacerbation due to 5-ASA intolerance. 5-ASA intolerance can be confusing and it can mislead physicians into considering unnecessary treatment escalation, including corticosteroid (CS), biologics, or even surgery. In spite of the clinical importance of 5-ASA intolerance, there have been few studies on its incidence, clinical features, and diagnosis. METHODS In order to evaluate the incidence, characteristic symptoms, disease course, and laboratory data of children with 5-ASA intolerance, we retrospectively reviewed the medical records of 80 children with UC. RESULTS Eleven of 80 children (13.8%) with UC were diagnosed with 5-ASA intolerance. The median time between the initiation of 5-ASA and the onset of 5-ASA intolerance was 10 days (range, 4-20 days) in patients not receiving CS. Drug-induced lymphocyte stimulation test (DLST) was performed in 10 patients, and was positive in eight. C-reactive protein (CRP) increased significantly when exacerbation of colitis symptoms occurred. CONCLUSIONS The incidence of 5-ASA intolerance was relatively high. Besides the challenge test, elevation of CRP and positive DLST appeared to support the diagnosis of 5-ASA intolerance.