Introduction Acute severe ulcerative colitis (ASUC) occurs in 15-25 % of all ulcerative colitis (UC) patients. Initial treatment with intravenous corticosteroids fails in 30-50 % of patients, for whom the next line of treatment is biological therapy or colectomy. Acute colectomy has a higher risk of morbidity and mortality than a scheduled colectomy. Data suggest that an accelerated administration of biological treatment in corticosteroid non-responders compared to clinical practice, 5-7 days with intravenous corticosteroids, may be superior in inducing disease remission, thus potentially avoiding acute colectomy. However, there are currently no patient friendly and objective diagnostic tool to preselect patients for such a treatment. The aim of this study is to examine if gastrointestinal ultrasound (GIUS) could preselect corticosteroid non-responders to biological treatment after 48 hours to increase effectiveness of the second line therapy and thereby reduce the morbidity and mortality of ASUC.||Methods and analysis The study is a clinician blinded observational multi-center study derived from the Department of Gastroenterology, Herlev Hospital, Denmark. Fifty ASUC patients will be included at the time of hospitalization and followed for 12 months. Baseline clinical activity scores, endoscopic scores, blood samples, fecal-calprotectin, vital parameters and GIUS measurements will be obtained prior to administration of intravenous corticosteroids. All examinations except fecal-calprotectin and endoscopy will be repeated at 48 ± 24 hours, 5-7 days and 3 months after treatment start. Endoscopic scores and fecal-calprotectin will be obtained after 3 months and an additional fecal-calprotectin after 6 ± 1 days. Treatment outcome will be registered at each event and after 12 months. Patients will be divided into corticosteroid responders and non-responders and compared to GIUS measurements at each event using non-parametric statistics (Mann-Whitney and Wilcoxon test) and time to endpoints by survival statistics (Kaplan Meier). ROC statistics will determine the best cutoff values for GIUS parameters for optimal sensitivity, specificity and accuracy.||Ethics and dissemination The study is approved by the National committee on health research ethics (H-18031264). Results will be published in relevant scientific journals and presented at international conferences. Fully anonymized data will be accessible from authors upon request.