BACKGROUND: 20-25% of patients with ulcerative colitis (UC) will experience at least one episode of acute severe ulcerative colitis (ASUC). Up to 20% of nonresponders to medical treatment require surgery during the index episode. For approximately one-third of the patients, ASUC is the first manifestation of the disease, presenting unique challenges. METHODS AND AIMS: Retrospective cohort study including 185 patients hospitalized with ASUC. We aim to compare the clinical outcomes between patients with new-onset and established UC. RESULTS: 62 (33.5%) patients had new-onset, while 123 (66.5%) had established UC. Demographic and baseline clinical characteristics were similar between groups. Patients with new-onset UC experienced longer hospitalizations (19 +/- 22 vs. 12 +/- 11 days; P = 0.002), longer delay until receiving rescue therapy [11 vs. 6 days (interquartile range : 6-8 days); P = 0.014] and more severe bacterial infections (16 vs. 4%; P = 0.005). We found no significant differences in steroid response (P = 0.719), need for rescue therapy (P = 0.522), response to rescue therapy (P = 0.234), colectomy (P = 0.655), mortality (P = 0.516), and hospital readmission (P = 0.929) between groups. After 1 year, colectomy or death occurred in 13.5% of patients. Independent predictors of adverse outcomes included active smoking, extensive colitis, the need for rescue therapy, and severe bacterial infections - but not new-onset disease. CONCLUSION: Major clinical outcomes were similar between patients with new-onset and established UC.