Objective To explore the clinical characteristics and risk factors of severe infectious mononucleosis（IM）.Method The clinical materials of 346 inpatients with IM received a retrospective analysis.A comparison was made between 305 common cases with IM and 21 cases with severe IM

and the risk factors of severe IM received Logistic regression analysis.Results Single-factor logistic regression analysis indicated that the items including hyperpyrexia（>40℃）

jaundice

the decreases in line two and line three hemogram

cardiac damage and the EBV DNA load（>5.0×106copise/mL） were related to severe IM（p<0.05 or p<0.01）.And the further Logistic regression analysis of these five factors showed that jaundice and the EBV DNA load（>5.0×106copise/mL） were relevant to severe IM（p<0.05 or p<0.01）.The regression equation is Logit（P）=-3.545+1.892x1+3.039x2

and its sensitivity was 57.1% and specificity 85.8%.Conclusion The EBV DNA load（>5.0×106copies/mL） and jaundice may be responsible for the occurrence of severe IM.