Infectious mononucleosis is an acute lymphoproliferative disease that is most common in children and young adults and is caused by Epstein-Barr virus.

Characteristic clinical feastures include : 1) fever, sore throat, and lymphadenopathy ; 2) an associated absolute lymphocytosis greater than 50%, of which at least 10% are atypical lymphocytes in the peripheral blood ; 3) development of transient heterophil and persistent antibody responses against Epstein-Barr virus ; and 4) abnormal liver function tests.

The most prevalent age of infectious mononucleosis in Korea was less than 5 years old and it has been very rare in adulthood. However, recently in Korea, possibly due to the increasing number of people who are non-immune to Epstein-Barr virus, the age range seems to be increasing from young childhood to over 10 years old and even to adulthood. We present a 19-year-old man who showed typical clinical features of infectious mononucleosis.