Gastrointestinal cytomegalovirus (CMV) infection is a common opportunistic infection in immunocompromised patients, especially patients with acquired immunodeficiency syndrome and transplant recipients. In contrast, CMV infection of the gastrointestinal tract is rare in immunocompetent individuals. We report a case of severe, protracted, and debilitating diarrhea caused by generalized CMV infection of the gastrointestinal tract in an elderly woman with no apparent immunosuppression. An extensive diagnostic investigation demonstrated CMV-associated disease affecting both the upper and lower gastrointestinal tracts (esophagus, small intestine, and colon). Such extensive simultaneous involvement of the alimentary tract in an immunocompetent patient is rare and presents a diagnostic and therapeutic challenge. The diagnosis was based on a combination of endoscopic, histopathological, serological, and polymerase chain reaction analysis findings and our patient was successfully treated with intravenous ganciclovir. Our case demonstrates that gastrointestinal CMV infection should be considered in the differential diagnosis of severe chronic diarrhea in immunocompetent patients and that antiviral treatment may be justified in this setting.