BACKGROUND: About 40% of patients with acute severe ulcerative colitis (ASUC) fail corticosteroid therapy, hence it is important to develop criteria which can predict steroid failure earlier. Our aim was to identify variables (clinical, biochemical and endoscopic) and develop a novel day 1 score for predicting steroid failure. METHODS: All admissions for ASUC (fulfilling Truelove and Witts Criteria) between January 1, 2015 and July 31, 2020 at GCUH and From January 1, 2018 to July 31, 2020 at LGH were retrospectively analysed. Review of electronic medical records was performed and clinical, endoscopic, laboratory data were collected. Steroid failure was defined as need for rescue therapy (medical or surgical). For comparisons of proportions, we used Pearson's Chi-square test or Fisher's exact tests. Quantitative data were compared using t-test or Wilcoxon rank sum test. To test for independent predictive factors, a logistic regression model was constructed with the requirement for rescue therapy as the dependent variable. RESULTS: There were 153 patients with 194 episodes of ASUC included. Seventy-seven (50.3%) female, median disease duration 1.8 years (0–6), 53 (27.3%) were index presentation of UC as ASUC. Forty-three (22.2%) episodes were on biological therapy at presentation (26 episodes on anti-TNF antagonists, 17 on Vedolizumab). Seventy-five (38.6%) episodes were on oral corticosteroids at admission. Eighty-eight (45.3%) episodes required rescue therapy [83 episodes received medical rescue (15 cyclosporine/68 Infliximab) and 5 underwent direct colectomy]. Seventeen (8.7%) episodes had a colectomy during the admission for ASUC. On univariate analysis of admission variables, oral steroids (OR 4.21, P < 0.001, CI 0.88-0.97), CRP (OR 1, P 0.005, CI 1.00-1.01), UCEIS score (OR 2.14, P < 0.001, CI 1.58–2.90) were significant for predicting steroid failure. Faecal calprotectin was not predictive of need for rescue therapy (OR 1, P 0.803). On multivariate regression analysis oral steroids at admission, albumin and UCEIS remained significant. We developed a novel score (ASUC score) allocating 1 point to each variable (S. albumin ≤ 30 g/L, Steroid use at admission, and UCEIS ≥ 7). 36/39(92.3%) patients with a score of ≥2 required rescue therapy (sensitivity 45.6%, specificity 96.7%, PPV 92.3%, NPV 67.4%, accuracy 73%); 43/132 patients (32.6%) of patients with a score of < 0.001, CI 3.14–9.29, AUROC 0.7756 and the need for colectomy during the same admission (OR 4.12, P < 0.001, CI 2.23–7.63, AUROC 0.8333). CONCLUSION: 92% of patients with ASUC score of ≥ 2 at admission (serum Albumin≤ 30 g/L, oral Steroid use, UCEIS ≥7 score) fail intravenous corticosteroid therapy and the risk of colectomy in this group is 3 times higher compared to the whole cohort; this group may benefit from upfront second-line therapy.