A 22-year-old man was admitted to our hospital with a 3-day history of left upper quadrant abdominal pain. Five days prior to admission, he had continuous fatigue, fever, and sore throat lasting for 1 week. Laboratory studies on admission showed a white blood cell count of 6,240/μL (normal range, 3,210 to 9,680) with 51% lymphocytes, 8% of which were atypical. A contrast-enhanced ultrasound sonography using Sonazoid (perfluorobutane; Daiichi Sankyo, Tokyo, Japan) revealed an enlarged spleen with hypoechoic wedgeshaped areas with absence of blood flow (Fig. 1A and 1B). A computed tomography (CT) scan of the abdomen revealed splenic enlargement with splenic infarcts (Fig. 1C). Acute Epstein-Barr virus (EBV) infection was confirmed by assessing serum EBV-DNA using polymerase chain reaction. Serum EBV-DNA was positive at the time of admission as well as 10 days later. From these observations, we concluded that the patient had infectious mononucleosis (IM) due to EBV infection with splenic infarction. Tests for thrombophilia were normal. After 16 days of conservative treatment, the patient was discharged without any complications. A repeat CT scan 2 months after discharge showed reduction of the infarct area accompanied by atrophic changes (Fig. 1D).