Background: Infectious mononucleosis (1M) is a self-limited disease, but a few cases may have severe complications. This retrospective study was to explore the epidemiologic and clinical characteristics of IM associated with Epstein-Barr virus infection (EBV-IM) in children. Methods: Hospitalized patients with EBV-IM were enrolled during January 2005 to October 2008 in Beijing Children's Hospital Affiliated to Capital Medical University. All patients were divided into four groups: <1 year (group I), 1 to 3 years (group II), 3 to 6 years (group III), and >= 6 years (group IV). The epidemiology and clinical characteristics were compared among the four groups. Results: Totally 418 patients were enrolled, with 245 boys and 173 girls. Fever, lymphadenopathy and pharyngitis were three main manifestations of the patients. The incidences of hepatomegaly, splenomegaly and rash were higher in the patients aged below 6 years, and with age increment the incidences lowered. In contrast, the patients aged <1 year had the lowest incidence of tonsillopharyngitis. The total white blood cell count was higher in the infantile group than in the other groups (P=0.038). The infantile group had significantly lower levels of serum alanine aminotransferase and aspartate aminotransferase than the older groups (P=0.007 and P=0.012 respectively). The percentage of CD4(+) T cell subset decreased and the percentage of CD8(+) T cell subset increased with age increment. Conclusions: The incidence of EBV-IM peaked in children at age of 4 to 6 years in Northern China. Most of the patients had the classic triad of fever, lymphadenopathy and pharyngitis. Clinical symptoms, signs, laboratory findings and complications of patients varied with ages. World J Pediatr 2011;7(1):45-49