Colitis from reactivation of established cytomegalovirus (CMV) colonization can complicate the clinical course in patients with an acute flare of ulcerative colitis (UC). Accurate and timely detection of active CMV infection or disease with appropriate anti-viral therapy may reduce complications associated with acute UC flare. Limited information is available on the presence of colonic CMV infection in patients with quiescent ulcerative colitis. Prospective studies on factors associated with reactivation of CMV infection during active UC flare and its impact on disease progression are lacking.