Objective To distinguish primary CMV infection from reactivation or reinfection. Methods A urea denaturation test was included in the washstep of the standard IgG ELISA in 12 patients of age＜12 whose serum CMV-IgG were positive, avidity indexes of CMV-IgG were measured. Results Among children aged 1～12 years, the varieties of avidity indexes of CMV-IgG in A group(IgM negative and IgG positive) and B group(IgM positive and IgG positive) had the same trend. There was only one case in B group whose avidity index was under 30%, the other were all above 30%, and most were above 50%. But among 40 children aged 1～12 months,the avidity indexes were under 30% in 21 cases,8 cases were 30～50% ,and 11 cases were above 50%. The antibody avidity in 1～6 months children was influenced by their mothers lgG,80% (12/15) of 7 ～12 months children had low-avidity antibodies. Conclusion CMV infection mostly occurred among 1～12 months children. Urea denaturation test was an efficient method to distinguish primary CMV infection from reactivation or reinfection.