Abstract Background Acute severe ulcerative colitis (ASUC) is a serious, potentially life-threatening condition. Hospital admission and prompt medical therapy with intravenous steroids is crucial, but rescue medical therapy is frequently needed, and surgery is often required. Our aim was to assess the adherence to main diagnostic and management recommendations within the MASC multinational study. Methods MASC was a prospective, multinational cohort study, including all consecutive patients hospitalized for ASUC between February 2019-January 2020. The primary outcome was surgery during the first 90 days after admission. We used descriptive statistics to determine the proportion of patients fulfilling each diagnostic and treatment recommendation according to ECCO and AGA. Results A total of 823 patients from 123 centers in 32 countries in five continents were included (median age 39 years [IQR, 28-54], 56% male, 64% non-smokers; Table 1). Most cases (74%) had a previous diagnosis of ulcerative colitis, and around half (48%) with prior ASUC episodes. Upon admission, they were under 5-ASA therapy (60%), thiopurines (14%), or biologics (21%). They were mostly admitted to the Gastroenterology ward (74%). During the initial assessment, 112 patients (15%) did not have stool culture or C. difficile testing, and CMV was evaluated in 50% of cases. Endoscopic assessment was performed in 66% of patients, mainly by flexible rectosigmoidoscopy (74%). Regarding treatment, 3.4% of patients received exclusive parenteral nutrition, 70% low molecular weight heparin, and 42% prophylactic antibiotics (Table 2). Steroids were started after a median of 0 days (IQR, 0-1), mostly with methylprednisolone or hydrocortisone (64%), 26% with other steroids, and 7% were managed without steroids. Infliximab was started in 159 patients (19%) after a median of 6 days (IQR, 4-8) after starting steroids, with 27% after >7 days. Most of the patients (93%) had some type of tuberculosis screening done prior to infliximab. Twenty-eight patients (3.4%) started cyclosporine. Thirty-two patients (4%) received other rescue therapy different from infliximab or cyclosporin. Colectomy was performed in 25 patients (3%). Conclusion The diagnostic and medical management of ASUC remains heterogeneous and different from current recommendations. This context demands more evidence and consensus on the best management of ASUC.