Objective : The golden standard of serodiagnosis of HFRS is IFA(indirect immunofluorescence antibody test). However, positive IFA for IgG antibody does not always correlate with current infection because IFA may be positive due to past exposure to Hantaan virus such as subclincal infection or due to vaccination in endemic area. So supplementary diagnostic method is needed to diagnosis of HFRS with more accuracy. Methods : In this study, the usefulness of IgG avidity assay with 8 mole/l urea in diagnosis of HFRS was investigated. Serum samples from 45 patients with acute phase HFRS and 79 residents of endemic area, who had HFRS antibody were tested for IgG avidity. Results : The distribution of IgG avidity index of HFRS antibody was different acute phase of HFRS patients from endemic area residents(p<0.001). Patients with acute phase HFRS exhibited lower avidity of Hantaan virus-specific IgG (mode of 64 of avidity index), in contrast endemic area resident had a higher avidity (mode of 4 of avidity index). Conclusion : The IgG avidity assay should assist in the diagnosis of acute phase HFRS and may be used to identify recent infection and past exposure to Hantaan virus.