Introduction: Biosimilar infliximab (CT‐P13; Inflectra, Hospira) has been shown to be safe, efficacious, and non‐inferior to originator infliximab (Remicade, Janssen‐Cilag) in the treatment of inflammatory bowel disease (IBD).1 Extrapolation to all indications within the treatment of IBD has been considered acceptable despite a lack of power to evaluate IBD subtypes. Data are lacking for biosimilar use in treating acute severe ulcerative colitis (ASUC). We aimed to compare the proportion of patients with ASUC who required dose acceleration and/or a colectomy by 30 days, be‐tween patients treated with CT‐P13 and Remicade. Methods: This was a single‐center retrospective review of inpatients with ASUC treated with infliximab. The hospital made a mandatory non‐medical switch to CT‐P13 on March 27, 2017. Patients with ASUC, as defined by the Truelove and Witts criteria, who required rescue therapy with infliximab be‐tween Day 3 and Day 5 as defined by the Oxford criteria were eligible for inclusion. Patients treated with Remicade from January 1, 2016, before the switch were compared with CT‐P13‐treated patients after the switch. The primary endpoints were (a) the proportion who required dose acceleration at Days 5‐7; (b) 30‐day colectomy rate; and (c) proportion who required dose acceleration or a colectomy by Day 30. Secondary endpoints included total hospital length of stay and time to colectomy. Patient demographics, Montreal classification, smoking status, and medication history were also ascertained. The primary endpoints were compared using Fisher's exact test for comparison of categorical variables. Time to colectomy was assessed using Kaplan‐Meier curves and compared using the log‐rank test. Results: There were 25 patients treated with Remicade and 18 with CT‐P13. The median age for Remicade and CT‐P13‐treated patients was 31 and 36 years, respectively. A total of 52% of the Remicade group were male compared with 78% in the CT‐P13 group. The median dose pre‐scribed was 400 mg in both groups. The majority in each group (60% vs 83%) had extensive (E3) disease. Most also never smoked (64% vs 78%) and were having their first presentation of ASUC (76% vs 78%). The primary and secondary outcomes are summarized in Table 1. Conclusions: No statistically significant differences were detected in any of the primary endpoints; however, the sample size was small and was likely underpowered to draw definitive conclusions. The trend to a significantly longer hospital length of stay and shorter median time to colectomy in the CT‐P13 group, however, warrant further study with a larger sample size. Reference 1 Jørgensen KK, Olsen IC, Goll GL et al. Switching from originator infliximab to biosimilar CT‐P13 compared with maintained treatment with originator infliximab (NOR‐SWITCH): a 52 week, randomised, double‐blind, non‐inferiority trial. Lancet 2017; 389: 2304‐16.