Objective:To summarize the clinical and laboratory characteristics of infectious mononucleosis (IM) in children.Methods:Clinical features and laboratory results of 270 cases with IM admitted to the Department of Pediatrics in Strategic Support Force Medical Center of People′s Liberation Army from January 2012 to December 2020 were retrospectively analyzed. χ2 test was used for comparison between groups. Results:IM mainly occurred in children aged 5 months to 18 years old in autumn and spring.The highest incidence rate (105 cases, 38.9%) was 3-<6 years old (preschoolers). There were 253 cases (93.7%) with fever, 266 cases (98.5%) with adenopharyngitis, 196 cases (72.6%) with tonsil pseudomembrane or exudation, 248 cases (91.9%) with cervical lymphadenopathy, 92 cases (34.1%) with eyelid edema, 202 cases (74.8%) with nasal obstruction, 124 cases (45.9%) with nasal obstruction and snoring, 24 cases (8.9%) with rash, and 112 cases (41.5%) with splenomegaly.A total of 225 cases (83.3%) presented with typical triplets of IM (fever, adenopharyngitis and cervical lymphadenopathy). Sixty-two IM patients were complicated with pulmonary infections and 3 cases with diarrhea.The main co-infection pathogens in children with IM were Mycoplasma pneumonia (MP) (79 cases, 29.3%), influenza A or B virus (34 cases, 12.6%), Streptococcus pneumonia (SP) (18 cases, 6.7%), adenovirus (22 cases, 8.1%) and cytomegalovirus (3 cases, 1.11%). A total of 46 cases (17.0%) had multiple infections.Laboratory test results suggested that absolute lymphocyte count ≥5.0×10 9/L was found in 199 cases (73.7%), and abnormal lymphocyte ratio >0.10 was found in 225 cases (83.3%). Some children had elevated transaminase levels.Epstein-Barr virus capsid antigen-immunoglobulin M (EBV-VCA-IgM) was positive in 249 cases (92.2%), Epstein-Barr virus capsid antigen-immunoglobulin G (EBV-VCA-IgG) was positive in 238 cases (88.1%), and Epstein-Barr virus nuclear antigen-immunoglobulin G (EBV-NA-IgG) was negative in all cases.EBV-VCA-IgG showed low affinity in all cases (<40%). EBV DNA tests of peripheral blood plasma were carried in 153 cases, of which 118 cases (77.1%) were positive. Conclusions:EBV related IM mainly attacks preschoolers.Most patients are presented with typical triplets of IM.Eyelid edema, nasal obstruction, snoring, splenomegaly and elevated transaminase levels are prevalent in IM children.Most cases have a favorable prognosis.