• Adult patients, age 18 years or older, with UC, who within 2 weeks have been started on mirikizumab therapy for moderate to severe UC or who plan to begin this therapy within the next 2 weeks. The start of the mirikizumab treatment must have been or be initiated in the setting of standard-of-care therapy.

• Anticipation that the patient will be followed by the participating center for the next 12 months.

• Diagnosis of UC must be established based on standard clinical, radiographic, endoscopic, and histologic criteria as described below.

⁃ Criteria for diagnosis of UC The following diagnostic criteria were developed by the NIDDK IBD Genetics Consortium and are provided as guidelines to complete documentation on individuals with Ulcerative Colitis.

⁃ A) Symptoms including one or more: diarrhea, rectal bleeding, abdominal pain, fever, extraintestinal manifestations, weight loss, or failure to thrive.

⁃ AND

⁃ B) Symptoms on two or more occasions separated by at least 8 weeks or ongoing symptoms of at least 6 weeks duration.

⁃ AND

⁃ C) Endoscopic Findings compatible with UC:

• Superficial inflammation and/or ulceration (involving only the mucosa and submucosa) of the colon, which is continuous from the rectum extending proximally without skip lesions or complete rectal sparing (relative rectal sparing is allowed for patients receiving topical, rectal therapy; patchiness of endoscopic inflammation may be observed in patients with partially treated ulcerative colitis).

• In patients with proctitis or left-sided ulcerative colitis, there may be an area of inflammation in the cecum, usually surrounding the appendiceal orifice.

• No inflammation of the small intestine (backwash ileitis is allowed - non-stenotic superficial inflammation of the terminal ileal mucosa associated with severe pancolitis, which resolves following medical or surgical treatment of the colitis).

• No features of Crohn's disease listed above. There is no minimum length of extension of UC required for inclusion in this study.