Cytomegalovirus (CMV) colitis has been reported infrequently in patients with underlying inflammatory bowel disease, and usually in those who are taking long-term corticosteroid drugs and who are potentially immunosuppressed. We report a 39-year-old man with long-standing ulcerative colitis, taking only sulfasalazine, who developed a viral prodrome followed by bloody diarrhea. Severe colitis was noted on endoscopy. He worsened with intravenous steroid and antibiotic drugs. Flexible sigmoidoscopy with biopsy before a contemplated colectomy revealed a moderately active colitis with large cells containing intranuclear inclusions scattered throughout the biopsies, characteristic of CMV colitis. Serologic and immunologic studies suggested recent CMV infection. The patient improved with intravenous ganciclovir and a steroid taper, and colectomy was avoided. CMV colitis is a potentially treatable cause of fulminant colitis.