Current therapies for ulcerative colitis (UC) fail to achieve satisfactory disease control. Selective inhibition of Janus kinase (JAK) type 1 may improve clinical outcomes in patients with UC while avoiding side effects associated with pan-JAK inhibition. The safety and efficacy of the selective JAK1 inhibitor ivarmacitinib (formerly SHR0302) were evaluated in patients with moderate-to-severe, active UC.