Cytomegalovirus (CMV) is one of the leading opportunistic pathogens affecting immunocompromised patients. We report a case of histologically-confirmed extensive CMV enterocolitis in a young woman after receiving rituximab and tocilizumab for the treatment of autoimmune encephalitis. During the antiviral treatment, she spontaneously excreted small intestinal casts per oral and colonic casts per anus. Even though intestinal cast is an extremely unusual condition, CMV infection should be included in the differential diagnosis.