Background/Aims: Despite the advent of advanced therapies, cases of so-called “difficult-to-treat” (D2T) ulcerative colitis (UC) persist. This study aims to clarify the epidemiological and clinical characteristics of patients with D2T UC. Methods: We conducted a nested case-control study using the Medical Data Vision Claims Database in patients with UC who began an advanced therapy (biologics, advanced small molecules, calcineurin inhibitors) from January 2018 through April 2023. D2T UC patients were defined as having 2 or more switches of advanced therapies, or as undergoing surgery for UC, within 2 years after the first advanced therapy. Results: Four hundred and one (16.7%) and 1,996 patients (83.3%) met the definitions of patients with D2T UC and non-D2T UC, respectively. After 1:1 matching by index year, 355 patients per group were included in the analysis. Multivariate logistic regression analyses, including sensitivity analyses based on follow-up period after the first advanced therapy, showed that a prescribed corticosteroid dose of ≥ 30 mg/day during the 6-month baseline period was associated with D2T UC. In D2T UC patients, median duration of the first advanced therapy was 99 days, and median number of advanced therapies per year was 1.7. The first advanced therapy was continued for 2 years in 78% of patients with non-D2T UC. Conclusions The proportion of D2T UC patients among UC patients starting advanced therapy was 16.7%. The factor most associated with D2T UC was the need for a corticosteroid dose ≥ 30 mg/day during the 6 months before initiation of advanced therapy.