Primary cytomegalovirus (CMV) infection is rare in immunocompetent adults, even rarer in elderly patients. Little is known about the severity of symptoms and the clinical course in this patient group. In children and younger adults, CMV mostly presents as an asymptomatic disease or a self-limiting mild mononucleosis-like syndrome. We describe the clinical course of an unusually severe primary CMV infection in a 69-y-old otherwise healthy man, as well as 6 other severe cases in immunocompetent adults at our institution, and compare them to adult cases from the literature. CMV primary infection and antiviral treatment should be considered in immunocompetent elderly persons presenting with a severe mononucleosis-like syndrome.