Toxic dilatation of the colon is sometimes a complication which occurs during exacerbations of inflammatory bowel disease. It often requires immediate surgical intervention. We describe here a male patient with known ulcerative colitis who was investigated for fever of unknown cause, diarrhoea, and general illness. Upon admission he had only modest symptoms of colitis and received no immunosuppressive treatment. The condition worsened considerably, and subsequent colonoscopy demonstrated widespread lesions compatible with viral disease, but not with ulcerative colitis. Antiviral treatment for cytomegalovirus was started immediately, but progression of the condition with toxic dilatation called for a rapid colectomy. Histological examination showed widespread lesions compatible with cytomegalovirus infection. This clinical picture is rarely seen in immunocompetent patients.