Haemorrhagic fever with renal syndrome (HFRS) caused by Hantaan viruses has been one of the principal acute febrile disease in Korea. To analysis the sero-epidemiological patterns of HFRS, 4,177 patient sera of acute febrile illness submitted for serological assay to National Institute of Health from Community Health Centers, Institutes of Health and Environment and hospitals from 1996 to 2005 were examined for antibodies against Hantaan virus by indirect immunofluorescent assay (IFA). Serum samples with greater than 1:32 antibody titer were considered positive. The results were analyzed seroepidemiologically by annual, sexual, seasonal, age and regional distribution of HFRS patients. Out of 4,177 serum samples tested, 1,415 samples (33.9%) were positive to Hantaan virus. The ratio of males (48.2%, 682/1,415) to females (38.2%, 541/1,415) was 1.3:1. Seasonal incidence showed that 69.5% (985/1,415) of cases occurred from October to December, resulting with higher prevalence in November (41.3%, 584/1,415). Regionally, seropositive rates of samples collected in Gyenggi, Gangwon and Chungbuk were 39.9% (564/1,415), 19.3% (274/1,415) and 8.5% (120/1,1415), respectively. Age distributions of seropositive of HFRS were detected from 20 to 79 years (78%).