Abstract: Objective:To investigate the clinical manifestations,laboratory examination and clinical features of liver damage in different age children with infectious mononucleosis(IM).Methods:Sixty IM children were divided into the age less than 6-year-old group and age more than or equal to 6-year-old group according to the average age of children(30 cases each group).The main symptoms and signs,laboratory test results and incidence rate of complications between two groups were compared using the unified clinical and laboratory indicators.The relationships between alanine aminotransferase(ALT)level and fever duration and children age in two groups were analyzed.Results:The varying degrees of symptoms and signs in two groups were found.The duration of feve and highest temperature in two groups were 3 to 31 d and 37.5 to 40.1℃,respectively,and the differences of those were not statistically significant(P>0.05).Among 60 cases,fever in 54 cases(90.00%),lymphadenopathy in 47 cases(78.33%),rash in 10 cases(16.67%) and angina in 40 cases(66.67%) were identified.The incidence rates of splenomegaly and hepatomegaly in age <6-year-old children group were higher,and the differences of those between two groups were statistically significant(P<0.05).The differences of the proportions of white blood cells,atypical lymphocytes and lymphocytes between two groups were not statistically significant(P>0.05).The levels of ALT and aspartate aminotransferase in age < 6-year-old children group were lower than those in age more than or equal to 6-year-old group(P<0.05).The varying degrees of complications(including liver damage,pneumonia,myocardial damage,anemia,thrombocytopenia and neutropenia) in two groups were found.Forty-seven cases with liver function demage(78.33%) in two groups were found,and the difference of which between two groups was statistically significant(P<0.05),the differences of other complications between two groups were not statistically significant(P>0.05).The ALT level was correlated with the duration of fever,and the longer the fever time and higher the abnormal rate,the degree of liver damage was the greater.The difference of the abnormal level of ALT in different age children was not statistically different(P>0.05).Conclusions:The clinical manifestations,laboratory tests and complication in differnent age IM children exist differences,and the liver dysfunction is the most common complication.Hepatosplenomegaly is common in preschool children,and liver dysfunction is common in older children.