Although most immune-related adverse events (irAEs) supplementary to immune checkpoint inhibitors can be managed with immunosuppressive therapies; they can induce reactivation of infectious diseases, including cytomegalovirus (CMV). bleeding and coarse mucosa in the rectum. b Sigmoidoscopy around the 6th hospitalization day demonstrated almost normal rectal mucosa. c Colonoscopy around the 12th day showed multiple punched-out ulcers in the descending colon. d Histologic examination of the ulcers showed crypt abscess and inflammatory cell infiltration (hematoxylin and eosin staining). Original magnification, 4. Scale ELF3 bar, 200 m. e Immunohistochemistry for cytomegalovirus was positive. Original magnification, 40. Scale bar, 20 m. f Double balloon enteroscopy exhibited multiple ulcers in the small intestine. Around the 6th day after hospitalization, sigmoidoscopy showed mucosal healing in the rectum (Fig. ?(Fig.2b).2b). Around the 7th day after hospitalization, methylprednisolone was reduced to 35 mg/day, but hematochezia and anemia were observed the following day. To confirm the source of bleeding, left-sided colonoscopy was performed around the 12th day after hospitalization and revealed multiple punched-out ulcers in the descending colon, but the remission in the sigmoid colon and rectum was maintained (Fig. ?(Fig.2c).2c). Histologic examination of the biopsy samples from the ulcers showed crypt abscesses and infiltration of lymphocytes and plasma cells (Fig. ?(Fig.2d).2d). Immunohistochemical study was positive for CMV (Fig. ?(Fig.2e),2e), although CMV pp65 antigenemia was unfavorable. He was diagnosed with CMV colitis, for which ganciclovir (5 mg/kg) twice daily was initiated. Methylprednisolone was tapered to 30 mg/day. Around the 27th day after hospitalization, Ezogabine cost total colonoscopy showed ulcers with healing propensity in the ascending and descending digestive tract. However, the anemia Ezogabine cost and hematochezia didn’t improve. To eliminate bleeding from the tiny intestine, video capsule endoscopy was performed in the 33rd time after hospitalization and uncovered active bleeding through the ileum. In the 36th time after hospitalization, dual balloon enteroscopy demonstrated ulcers with recovery propensity in the ileum (Fig. ?(Fig.2f).2f). Although CMV had not been discovered through the biopsy examples of ileal ulcers as of this correct period, this healing propensity could be because of the antiviral therapy. Thereafter, there is no recurrence from the hematochezia, as well as the anemia gradually improved. In the 40th time after hospitalization, ganciclovir was gradually discontinued and steroids were tapered. He was discharged in the 69th time after hospitalization. Dialogue We herein record a complete case of CMV enterocolitis under corticosteroid therapy for irColitis. In irColitis with corticosteroid-refractory behavior, total colonoscopy and histologic evaluation ought to be executed to eliminate CMV reactivation. In this case, a biopsy of colon ulcer provided the definite Ezogabine cost diagnosis of CMV enterocolitis, and antiviral therapy improved enterocolitis without augmenting immunosuppressive therapy. Cancer immunotherapy with ICIs has revolutionized the treatment of various types of advanced cancer [1, 2, 3]. Although ICIs can afford clinical benefits, they may cause several side effects called irAEs  that involve any organs, including the colon. irColitis causes diarrhea, bloody stool, abdominal pain, high Ezogabine cost fever, and, in some cases, colon perforation that can be a life-threatening event. The incidence of diarrhea had been reported to be 12.1C13.7% for anti-PD-1 and 30.2C35.4% for anti-CTLA-4, whereas the incidence of colitis had been reported to be 0.7C1.6% for anti-PD-1, 5.7C9.1% Ezogabine cost for anti-CTLA-4, and 13.6% for the combination of both agents . The endoscopic findings of irColitis closely resemble those of ulcerative colitis and include erythema, loss of vascular pattern, erosions, ulcerations, and luminal bleeding . In most cases the sigmoid colon and rectum are involved, and therefore sigmoidoscopy had been usually sufficient to make a diagnosis of irColitis, and total colonoscopy can be avoided to decrease the risk of perforation . According to this evidence, we performed sigmoidoscopy at the onset of diarrhea, and endoscopic findings led to the diagnosis of irColitis. Histologically, acute colitis can present as mononuclear cell.