Infection by cytomegalovirus (CMV) in immunocompetent patients is rare, and if it occurs it is most often associated with ulcerative colitis. This case illustrates a CMV infection in a patient with an ulcerative colitis combined with CMV-induced protein losing enteropathy, a condition reported in immunocompetent individuals in only a very few cases worldwide. It demonstrates the importance of differentiating between a flare-up of ulcerative colitis and CMV colitis. The indication for antiviral therapy is discussed. A 76-years-old patient with a 23-year history of leftsided ulcerative colitis presented with acute pancolitis sparing the rectum. He showed no evidence of impaired host defence, nor has he ever had taken immunosuppressive drugs. Disseminated primary CMV infection involving of the colon, the oesophagus and the small intestine with protein losing enteropathy was diagnosed on the basis of histology, culture and serology. In view of the long duration of the illness and the highly active infection, antiviral therapy with ganciclovir was given and led to a dramatical improvement of all disease manifestations. The patient subsequently remained in remission from ulcerative colitis for three years.