We propose a pilot study investigating the treatment of severe ulcerative colitis with high-dose infliximab (10mg/kg infusions), which is the standard of care at UCSF for severe, hospitalized ulcerative colitis. This will be a prospective, multi-site study which will consecutively enroll hospitalized ulcerative colitis patients meeting inclusion criteria from anyone of four UCSF-affiliated hospitals. Per routine care, enrolled subjects will undergo infectious work-up, receive intravenous corticosteroids in addition to supportive care, and undergo evaluation by a Colorectal Surgeon. Subjects that are eligible for anti-TNF therapy after routine screening will begin infliximab induction therapy on hospital day 3 at 10mg/kg. We will perform standardized pre- and post-treatment assessments of disease activity (Ulcerative Colitis Disease Activity Index - UCDAI), including acquisition of biochemical and endoscopic data. We will measure serum levels of tumor necrosis factor alpha, infliximab, and infliximab antibody throughout the course of treatment. The primary outcome will be avoidance of colectomy. This study will be the first to our knowledge to examine the empiric use of high-dose infliximab combined with early therapeutic drug monitoring to guide the care of patients hospitalized with severe ulcerative colitis. Our ultimate goal is to develop a rational approach to maximizing infliximab therapy in the setting of severe ulcerative colitis to minimize and predict primary non-response to infliximab induction therapy.