【Abstract】Objectives The aims of this research were to quantitatively measure copy numbers of EBVBALF5 gene in peripheral blood mononuclear cells from children with infectious mononucleosis and to correlate copy numbers with clinical features.Methods 80 children with newly diagnosed infectious mononucleosis and 40 healthy control children were enrolled in this study. EBVBALF5 copy numbers in mononuclear cells from IM patients and control children were determined by absolute fluorescence quantitative PCR and correlate copy numbers with ALT, AST, LDH, WBC and percentage of atypical lymphocytes. Results EBVBALF5 gene was detected in 73 children with IM, with copy numbers ranging from 10.3~10.5 copies/ml (median copy number 2.4(10.4copies/ml).EBVBALF5 gene was also amplified in 2 control children, but with much lower copy numbers. Rank correlation test revealed that copy numbers in IM group were statistically higher than that in control children. Samples were divided into high copy number group(arbitrarily defined as copy numbers(1(10.4copies/ml) and low copy number group(arbitrarily defined as copy number＜1(10.4copies/ml).The probability of liver function abnormality in higher copy number group was statistically higher than that in low copy number group（P＜0.05）. Ranked correlation test revealed that EBVBALF5 copy numbers were positively correlated with increment of ALT, AST and LDH, but were not correlated with total WBC count and percentage of atypical lymphocytes in the peripheral blood. Conclusions EBVBALF5 gene load could be reliably measured by absolute fluorescence quantitative PCR. Copy number of EBVBALF5 gene is correlated to the likelihood of organ function damage such as liver enzyme increase.