Cytomegalovirus (CMV) is a prevalent pathogen, with 98∼100% of Korean adults showing prior exposure by serology. A primary infection, such as CMV infectious mononucleosis, is very rare. CMV infectious mononucleosis often presents an initial diagnostic problem. Patients are often hospitalized with a wide variety of clinical diagnoses including fever of unknown origin without pharyngitis and lymphadenopathy. CMV gastrointestinal infections are rare in previously immunocompetent individuals. The most common sites involved are the colon and rectum, although lesions of the stomach have also been described. It is unusual to see CMV infectious mononucleosis and CMV gastrointestinal infection in the same patient. Our patient received symptomatic treatment and fully recovered. We present a case of CMV infectious mononucleosis with gastric ulcers in a previously healthy adult.